Teacher: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.
Teacher: Now, understand the problem? Solve this instance: Context: Dave had 24 files and 13 apps on his phone. After deleting some apps and files he had 17 apps and 21 files left. 

 Question: How many files did he delete?
Student:
3