Julian bought some files at $2 each. He later sold them to his customers at $4 each. For customers who bought two files were given another file for free. The customers bought either one or two files. Julian gave away 220 free files and collected a total of $560 more than what he had paid for. How many customers bought just one file each?

	Files sold	Files given to customers who got 1 file each	Files given to customers who got 3 files each
Number	1 u	1 u - 660	660
Value	$2	$4	$4
Total Value	2 u	4 u - $2640	$1760

Since 220 files were given free, there were 220 customers who bought two files each.

Number of files that each customer would get if they bought two files each and got 1 file free
= 2 + 1
= 3

Total number of files for customers who each bought two files 
= 3 x 220
= 660

Number of files for customers who each bought 1 file = 1 u - 660

Total amount collected from customers who bought 1 file each
= 4 x (1 u - 660)
= 4 u - 2640

Total amount collected from customers who bought two files each
= 220 x 2 x 4
= $1760

Total amount collected from all the customers
= 4 u - 2640 + 1760
= 4 u - 880

Julian collected a total of $560 more than what he had paid for.

4 u - 880 = 2 u + 560
4 u - 2 u = 560 + 880
2 u = 1440

1 u = 1440 ÷ 2 = 720

Number of customers who bought just one file
= 1 u - 660
= 720 - 660
= 60

Answer(s): 60