You are given a math word problem and you are supposed to apply a single mathematical operator 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.

[EX Q]: a chef used 15 apples to make a pie . now he has 4 apples left . how many apples did he have before he made the pie ?
[EX A]: 19

[EX Q]: lucy put 68 shells in her bucket . she found some more shells and now she has 89 shells . how many more did she find ?
[EX A]: 21

[EX Q]: there are 13 different books in the 'crazy silly school ' series . if you read 9 of the books how many more do you still have to read ?
[EX A]:
4