A chef had 31 apples and 8 peaches. After making some pies, he had 47 apples left. How many apples did he use?
The chef started with 31 apples and 8 peaches. 
Then, he made some pies. 
After making the pies, he had 47 apples left. 

So, we can say that the chef used 31 - 47 = -16 apples.