A recipe for cookies calls for 3 1/4 cups of sugar. And you have already put 3 1/9 cups. How many more cups do you need to put in?

Question

A recipe for cookies call for 3 1/4 cups of sugar. Amy has already put in 3 1/9 cups. how many more cups does she need 

Answer 1

Ashley, we obviously need to subtract 3 1/4 - 3 1/9.  We can leave the 3 out for now and just subtract 1/4 - 1/9.

 

First, we need to find a common denominator.  We must find the LCM of 4 and 9.

 

4, 8, 12, 16, 20, 24, 28, 32, 36, 40

 

9, 18, 27, 36, 45

 

The LCM is 36.

 

1/4 = ?/36.  Since 4 x 9 = 36, we must multiply 1 x 9 to get 9/36.  We do the same with 1/9.  1/9 = ?/36.  9 x 4 = 36. 1 x 4 = 4, so we have 4/36.

 

9/36 - 4/36 = 5/36

 

We need 5/36 of a cup!

 

Hope this helps!