Mark bought some files at $2 each. He later sold them to his customers at $5 each. For customers who bought two files were given another file for free. The customers bought either one or two files. Mark gave away 160 free files and collected a total of $1270 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 - 480	480
Value	$2	$5	$5
Total Value	2 u	5 u - $2400	$1600

Since 160 files were given free, there were 160 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 160
= 480

Number of files for customers who each bought 1 file = 1 u - 480

Total amount collected from customers who bought 1 file each
= 5 x (1 u - 480)
= 5 u - 2400

Total amount collected from customers who bought two files each
= 160 x 2 x 5
= $1600

Total amount collected from all the customers
= 5 u - 2400 + 1600
= 5 u - 800

Mark collected a total of $1270 more than what he had paid for.

5 u - 800 = 2 u + 1270
5 u - 2 u = 1270 + 800
3 u = 2070

1 u = 2070 ÷ 3 = 690

Number of customers who bought just one file
= 1 u - 480
= 690 - 480
= 210

Answer(s): 210