M3.3.2 For the adult
Adding and taking away fractions
How can I add a third and a quarter when the parts are different sizes?
What this chapter teaches
Fractions can only be added when their parts are the same size. The learner changes both fractions to a common bottom number, usually the lowest common multiple, and then adds or takes away the tops. With mixed numbers, one whole may need to change into parts.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why can you not add 1/3 and 1/4 by adding the tops and adding the bottoms?
- How do you find a common bottom number for 4 and 6?
- In 3 1/4 − 1 1/2, what do you do when the quarters are too few to take away?
After the chapter, the learner can
- Add 1/3 + 1/4 by finding a common bottom number
- Take away mixed numbers, like 2 1/2 − 1 3/4
What they copy into their notebook
Parts must be the same size before you add. Find the LCM of the bottoms, change both fractions, then add the tops. The bottom stays.