There were w dollars in Olivia's wallet. She collected x more dollars from an atm. After she visited a supermarket there were y dollars left. How much more money did she spend at the supermarket than she collected at the atm?
Olivia had w dollars in her wallet. 

She collected x more dollars from the ATM, so now she has w + x dollars. 

She spent some money at the supermarket, so now she has w + x - y dollars. 

We want to know how much money she spent at the supermarket, which is equal to y.