Detailed Instructions: You are given a math word problem and you are supposed to apply multiple mathematical operators like addition, subtraction, multiplication or division on the numbers embedded in the text to answer the following question and then only report the final numerical answer.
Q: A waiter had 21 customers in his section . If 12 of them left and the rest of his tables had 3 people at each table , how many tables did he have ?
A:
3