You are given a math word problem and you are supposed to only use subtraction on the numbers embedded in the text to answer the following question and then only report the final numerical answer.
--------
Question: Context: A waiter had 13 customers. After some left he got 4 new customers. If he still had 9 customers

Question: How many customers left?

Answer: 8

Question: Context: There are 20 different books in the ' crazy silly school ' series. If you are yet to read 5 of the books

Question: How many books have you already read?

Answer: 15

Question: Context: David did 30 more push-ups than Zachary in gym class today. If David did 37 push-ups

Question: How many push-ups did Zachary do?

Answer:
7