Paco had 29 cookies. He ate some of them and had 12 cookies left. How many cookies did Paco eat?
Paco had 29 cookies. 
He ate some of them. 
He had 12 cookies left. 

We know that Paco started with 29 cookies, and we know that he ended with 12 cookies. So, we can say that Paco ate 17 cookies.