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?

We are told that Paco had w salty cookies and x sweet cookies. He ate y sweet cookies and z salty cookies. The question is asking how many salty cookies Paco had left. 

We can set up an equation to solve this problem. 

w + x = the total number of cookies Paco had
y + z = the total number of cookies Paco ate

If we subtract y + z from both sides of the equation, we get: 

w + x - y - z = the total number of cookies Paco had - the total number of cookies Paco ate 

w + x - y - z = the number of cookies Paco had left

Therefore, the answer to the question is w + x - y - z.