How often have you taught fraction division to your students only to find them "flipping" the wrong number? You may have taught them to "skip, flip, flip," "invert and multiply," or "multiply by the reciprocal." You may have listed out the steps, or taught them a nifty song, but somehow they still flip the wrong one or they forget to flip at all.
OR they change a mixed number into an improper fraction and seem to subconsciously think that since they did something to that mixed number, the flipping had already occurred...and then they don't flip anything.
Why does this happen? I'm going to say that it happens because they don't see the sense in it - it doesn't mean anything to them.
So, I have another way to teach fraction division - perhaps you've heard of it, or you use it. I never learned it this way as a child, but I like it and it makes more sense to some students. I learned this method when I had a student teacher a few years back. She was teaching the fraction unit, and when her supervisor came in to observe and discuss, she asked if I had ever taught fraction division using common denominators. Having only learned (and then taught) to multiply by the reciprocal, of course I said no.
The next time she visited, she brought me a page from a textbook that explained dividing fractions using common denominators. These are the steps:
Step 1: Find common denominators, just as when adding and subtracting and then make equivalent fractions (students are already used to doing this - hopefully).
Step 2: Create a new fraction with the numerator of the first fraction over the numerator of the second fraction...this is your answer.
Done (unless you need to reduce)!
I was shocked - it seemed SO simple!
Check out this example - it's a simple one, for starters:
5/6 divided by 2/3.
1) Find the common denominator of 6 and 3, which is 6. This gives you 5/6 divided by 4/6.
2) The first numerator (5) becomes the numerator in the answer. The second numerator (4) becomes the denominator. Then reduce.
Let's look at another one, with mixed numbers:
1 and 4/7 divided by 1 and 3/4.
1) Convert the mixed numbers to improper fractions, which gives you 11/7 divided by 7/4.
2) Find the common denominator of 28 and make equivalent fractions. This gives you 44/28 divided by 49/28.
2) The first numerator (44) becomes the numerator in the answer. The second numerator (49) becomes the denominator. No reducing, in this case.
I've shown both methods to my sixth-graders. Some really like it. Others stick to the flipping method - but I don't know if this is because they like it better or because it was the first way they learned it.....most of them had been taught something about fraction division in 5th grade.
As far as teaching multiplying by the reciprocal - if students are going to use it, I think it's important that they understand WHY it works. It may be tough for them to understand, but if they learn the common denominator method first, the proof may then make more sense to them. I found a great article on the NCTM website that uses the common denominator method to prove why multiplying by the reciprocal works - check it out!
Recently I made two math wheels, to use to teach both methods of dividing fractions -taking notes will be more fun!
What do you think? Do you see any advantages or disadvantages to teaching fraction division using common denominators?
I love teaching fraction multiplication--particularly multiplication of mixed numbers. Why? Because I have fun explaining why multiplying mixed numbers DOESN'T work a certain way.
Inevitably, when we start multiplying mixed numbers, some students want to multiply the fractions by the fractions and then multiply the whole numbers by the whole numbers. And I can see why they might think that's ok - after all, when you add and subtract, you deal with the whole numbers and fractions separately. Sometimes, I think they don't want to be bothered with making improper fractions, because it's "easier" to just do 2 x 3 and then 3/4 x 1/2, haha.
So, every year, we end up having this discussion about why that just doesn't work. I enjoy showing/explaining that multiplying 2 3/4 by 3 1/2 means that ALL parts of 2 3/4 must be multiplied by ALL parts of 3 1/2. On the board, we make a list of the problems that would need to be completed: 2 x 3, 2 x 1/2, 3/4 x 3, and 3/4 x 1/2.
Now that we have all four products, we go through the process of adding them all together (finding common denominators, equivalent fractions,etc) and then reducing.....quite a bit of work to get to the answer:-)
Then we compare that to what we get when we convert the mixed numbers to improper fractions. The detailed example of completing four multiplication problems and adding the products proves not only that converting to improper fractions is necessary, but also that it's a lot faster! So, Tip #1 is to show students WHY what they're doing isn't correct...show what the multiplication really means.This may also mean bringing out the graph paper and showing what 3/4 groups of 1/2 looks like, etc, in addition to doing the computation.
Canceling, or "Crossing Out"
Tip #2 - Teach students to cancel, or "cross out" (or whatever you might call it), and show them why it makes life a little easier.
I love teaching this aspect of fraction multiplication. It's hard for some students to grasp at first, but when they repeatedly see that if they don't cross out, they have to reduce at the end of the problem (with larger numbers, like 168/12), they start getting excited about finding how much they can cross out. In recent years, I've found that students aren't learning this in earlier grades as often as they used to--for many, the discussion we have in my classroom is the first time they've encountered it.
Once I teach them the idea of reducing first, and we explore why it works, there are some that still want to stick with what they learned in earlier grades and reduce only at the end, while others get super-excited about the concept of making the numbers they're working with smaller at the start. I may be wrong on this, but it seems that the students who embrace it first are those who know their multiplication facts better and can more easily find the relationships between the numbers in the problem....a student who knows that 15 and 24 can both be divided by 3, for example, is more likely to go ahead and divide them by 3 than the student who can't see it because they can't remember/don't know what 15 and 24 are divisible by.
Multiplying Fractions and Mixed Numbers Wheel
Tip #3 - Give students a graphic organizer to help them remember the process. Some will need this and some won't, but it's handy to have in their binders to reference throughout the year. I recently created a fun math wheel, which is a great way to have students take notes about the concept, practice it, and then add their own personal, artistic touches.
Do you have any special methods you use to teach the multiplication of fractions and mixed numbers?
This is a post I wrote back in 2013 (now revised), on my other blog, so the observation I refer to was quite a while ago now...how time flies!
I was observed by one of my assistant principals today (a Friday). After 20 years, I don't get super-worried when I'm going to be observed, but I still feel a little anxious. Today, I decided to have the students complete a problem solving activity and then start a "Footloose" activity, even though they wouldn't finish....Footloose normally takes about 40 minutes, so I figured they could do about half and then finish on Monday. (I do this fairly often, to give students flexibility in their work time - they can take as long as needed to complete problem solving, but if they get done quickly, they can move on). Things went so well during the observation...AP commented that there was so much going on in the room, and that the kids were so engaged! I was happy:)
During the class, students worked on group problem solving, (which they have done previously, with other math skills). These particular problems involved comparing and ordering fractions. Our procedure was as follows:
1) Each group received a different sheet with a problem "situation" and 3-4 questions about that situation. (I have five different sheets so that we can do the problem solving several different days with the same concepts, if needed and if time allows).
2) Each group read their situation and each of the questions together.
3) Each student spent 5-7 minutes, thinking/working individually to solve the questions, writing their work on their own recording sheet.
4) When students completed their individual thinking time, they compared their ideas (and answers if they had them), discussed any differences in thought, and worked to agree on final answers.
5) The final answers (with work) were written onto a group answer sheet to hand in.
When we did this type of group problem solving the first time (with decimal problems), we spent about 5 days on the problem solving, with each group working on a different problem sheet each day. The students really like the problem solving, partly because they are able to talk out their answers with each other. It's great to hear their communication about math and how they are able to point out the steps a group member needs to complete or the concepts that they may have missed.
Today, it was great to hear them say "Oh, we're doing this again. I like this!" My AP commented that he listened to hear what they were talking about, to see if they were focused, and he could hear one student explain to another how the work that they had done was different from another student.
The problem solving took about 15 minutes, and then as each group finished their problem, they moved on to Comparing and Ordering Fractions Footloose. This is a great game for keeping students engaged, but moving! Students start out with one card and a sheet of paper with 30 blank "blocks" in which to write answers to the questions on the cards. Each card has a number on it, and students record the answer to each card in the same number block as the number on the card. After answering the question on the card they start with, students put the card on the chalk ledge and pick up another card with another question to answer. Students continue answering and returning cards until they have answered all 30 questions. Students work so quietly when they are doing this activity! My AP said it was like "night and day" when they switched from the problem solving to Footloose - they were talking about the p.s., but as soon as they started the Footloose, it was sooo quiet.....and I didn't have to say anything for it to be this way - it just happened.
As I mentioned, I don't really get worried when an observation comes around, but it was great to hear the positive feedback for these activities that I create for my students!
I'm really liking the math wheel idea, so I created a new wheel for fraction, decimal percent conversions:-)
How to use this resource (this information is also in the free download):
Around the outside of the wheel are the different conversion headings – you can use the wheel to introduce the conversions, filling in just the ones you are covering each day. Or, you can use it to review all the conversions at once. In either case, the wheel can be kept in students’ notebooks as a reference/study tool.
1) I like to begin with decimal to percent and percent to decimal. In the arrows in these sections, you’ll see x 100 and ÷ 100. It think it’s important that students understand that these are the operations being used for these conversions before giving them a shortcut, so I let them use calculators to complete the examples. Once the examples are complete, I ask the students to look for the pattern – what happens to the decimal point in each of these cases?
We decide on the “shortcut” rules together and then write them at the bottom of those sections.
2) The fraction to percent and fraction to decimal sections have the rules written already, so the examples just need to be completed.
I always relate fraction to percent to students grades. By the time we get to this topic during the year, students have been figuring out their grades for months (I never write their percentages on their assessments – they need to calculate
them). They know how to find their percentage if their quiz grade was 6/8 or their test was 48/52. However, sometimes they need a reminder that this official fraction to percent “rule” is the same thing they’ve been doing for months! I have them
write a little reminder in that section - “just like test grades!”
3) For percent to fraction, students need to remember that percent means “out of 100,” so the percent number will always go over 100. Then they must reduce.
4) I find that decimal to fraction is sometimes tricky for students. When they have trouble, I ask them to read the decimal number according to place value (“How do you say this number, using tenths, hundredths, or thousandths, etc.?”). Once
they speak it, they know how to write the fraction – 0.27 is 27 hundredths, which is 27/100. After completing the examples, we discuss the idea that the denominator will be whatever the last decimal place is (10, 100, 1000, etc.) and the numerator will be the digits in the decimal number. We write this rule as simply as possible.
5) Students then complete the 10 problems around the page. Above each number is the conversion to complete (F to P, P to D, etc.) They can then color the rest of the wheel background.
I had a great time coloring my answer key! These could make a fun decoration as well:-)
I hope you can use it!!
Do your middle school math students like to play math games? Mine do, but over the past few years I've noticed that many of them aren't familiar with some of the games I played when I was a kid, like Yahtzee, for example. So, as we started working on converting fractions and decimals, I decided to create a game to make practicing the conversions more fun AND give them some more game experience! I based it on the idea of Yahtzee:-)
Here's how it works:
Students roll four dice, and pair the dice up to create "target numbers" that are either decimals or whole numbers.
For example, a student rolls 1, 2, 4, and 6. From these dice, the student may create any two of the following decimal (or whole) numbers:
½ = 0.5 4/1 = 4
¼ = 0.25 4/2 = 2
1/6 = 0.1666... 4/6 = 0.666...
2/1 = 2 6/1 = 6
2/4 = 0.5 6/2 = 3
2/6 = 0.333... 6/4 = 1.5
Once a player has chosen two target numbers, he or she finds the score by adding the dice that were used for each decimal (or whole number). If the player chose to use 1 and 4 to get 0.25, he or she adds 1 + 4 for a sum of 5 to place in the score column. If the second choice used 2 and 6, to equal either 0.333...or 3, then sum of 8 would go in the appropriate column as the score.
On the next roll, this student rolls 1, 1, 3, and 5. This student can pair 1 and 1, to get 1, and pair 3 and 5 to get either 0.6 or 1.666... The score for 1 is 2 (1 + 1) and the score for 0.6 (or 1.666) is
8 (3 + 5).
In many cases, students' scores will be the same, but some of the decimals can be found with different combinations of numbers (1 and 3 = 0.333..., and so do
2 and 6, so students could have a score of either 4 or 8). Some students will notice this sum difference and go for the combination that will give them the higher score....bringing in the possibility of using some strategy, for those higher level thinkers.
The students have really enjoyed playing this game. They do need a few examples at the start, to understand exactly how the game works, so if you decide to try the game, be prepared to go through a few turns together.
You can create a score sheet like this on your own, or go to TPT and use what I've created. Detailed instructions are included, and a complete answer key of highest and lowest possible scores for each target number are included as well. This is handy to quickly check student score cards as you check in on their games.
If you give it a try, please let me know how it goes!
Hi, I'm Ellie! I've been in education for 25 years, teaching all subject areas at both the elementary and middle school levels.