Answer:
0.75 or 3/4 cups 
Explanation:
Make a proportion 
We need 1 1/4, or 1.25 cups of sugar for 20 cookies, and we need x cups of sugar for 12 cookies 
1.25/20=x/12
To solve, we need to get the variable,x, by itself. It is being divided by 12. To reverse this, multiply both sides by 12. 
12(1.25/20)=(x/12)12
The 12s on the right will cancel, and we are left with 
0.75=x 
0.75 is equivalent to 3/4 
So, 0.75 or 3/4 cups of sugar is needed to make 12 cookies