After Olivia visited a supermarket there were 29 dollars left. If there were 42 dollars in her wallet initially How much did she spend?
Olivia has 42 dollars in her wallet. 
She spends some of it at the supermarket. 
Now she has 29 dollars left. 

How much did she spend?

We can set up an equation to solve this problem: 

42 - x = 29

x = 13

Olivia spent 13 dollars at the supermarket.