Question

A recipe for cookies calls for 3 1/4 cups of sugar. Amy has already put in 3 1/9 cups. How many more cups does she need to put in?

Answer 1

The answer in 5/36 of a cup

Answer 2

So 3 and 1/4 is equal to 4/4+4/4+4/4+1/4=13/4

so she put in 3 and 1/9 or 9/9+9/9+9/9+1/9 or 28/9

28/9+x=13/4
13/4-28/9=x

so we convert so that the bottom numbers are the same

a short cut is 3 and 1/4+x=3 and 1/9 
3 1/9+x=3 1/4
subtract 3 from both sides
1/9+x=1/4
subtract 1/4 from both sides
x=1/4-1/9
convert bottom number to same thing
1/9=4/36
1/4=9/36
x=9/36-4/36
x=5/36
5/36 cups