Detailed Instructions: You are given a math word problem and you are supposed to apply addition or subtraction mathematical operators on the numbers embedded in the text to answer the following question and then only report the final numerical answer.
Problem:A waiter at ' The Greasy Spoon ' restaurant had 39 customers to wait on . During the lunch rush he added another 12 customers . If 49 of the customers did n't leave him a tip , how many customers did leave a tip ?
Solution:
2