'Buy two get one free' Question

Question:

Mr Lee bought some files at $4 each and sold them at $10 each. The customers who bought 2 files from him were given 1 file for free. Yesterday, all his customers bought either one or two files. At the end of the day, he had given away 120 files and had earned $1230. Find the number of customers who bought only one file. 

Solution: