Paco had w salty cookies and x sweet cookies. He ate y sweet cookies and z salty cookies. How many salty cookies did Paco have left?
Paco had w salty cookies and x sweet cookies. This means that the total number of cookies Paco had is w + x. 

Paco ate y sweet cookies and z salty cookies. This means that the total number of cookies Paco ate is y + z. 

Therefore, the number of salty cookies Paco had left is w - z.