Student: When I add fractions that have the same
denominator, such as 1/5 + 2/5, I know that the answer is 3/5. But what do I do when the denominators are
Mentor: The first thing is to find the lowest common denominator, the smallest number that both
denominators divide into evenly. If I am adding 3/4 + 1/6, what is the lowest common
Student: Well, it looks to me like it would be 12. Both 4 and 6 can divide into 12.
Mentor: Exactly! So you rewrite the problem to look like this:
Instead of 3/4 + 1/6, write it as:
something/12 + something/12. Then you will see what the correct answer is.
To find out the "somethings", start with the first fraction. If you are changing the
denominator in the fraction 3/4 to "something"/12, you multiplied 4 by 3 to get 12. So
numerator by 3 also. That gives us 9/12.
Student: OK, now I'll try the second fraction, 1/6. If I change the denominator to 12, I multiplied 6
by 2 to get 12. So I will multiply the numerator (1) by 2, which is 2. So that gives me a
fraction of 2/12.
Mentor: Right so far. We just made the problem easier to solve by converting it this way:
3/4 + 1/6 became 9/12 + 2/12
Student: That's much easier to solve. 9 + 2 = 11, so the answer would be 11/12.
Mentor: Yes! It works the same way for subtraction. Try this problem: 4/5 - 4/15.
Student: Let's see. The lowest common denominator is 15, because both 5 and 15 divide evenly into it.
So I can rewrite the problem:
4/5 - 4/15
something/15 - something/15
I'll start with the first fraction. I multiplied the denominator 5 by 3 to get 15, so I will
also multiply the numerator, 4, by 3. That will make the fraction 12/15.
The second fraction is very easy. The denominator is already 15 (15 divides into 15 one time),
so I will multiply the numerator by one, giving me a fraction of 4/15. My subtraction problem
now looks like: