Definition: 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.
Input: Context: Zachary did 15 push-ups in gym class today. David did 39 more push-ups than Zachary. John did 9 push-ups less than David.

Question: How many more push-ups did Zachary do than John?
Output:
30