Glen bought some files at $4 each. He later sold them to his customers at $6 each. For customers who bought two files were given another file for free. The customers bought either one or two files. Glen gave away 230 free files and collected a total of $80 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 - 690	690
Value	$4	$6	$6
Total Value	4 u	6 u - $4140	$2760

Since 230 files were given free, there were 230 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 230
= 690

Number of files for customers who each bought 1 file = 1 u - 690

Total amount collected from customers who bought 1 file each
= 6 x (1 u - 690)
= 6 u - 4140

Total amount collected from customers who bought two files each
= 230 x 2 x 6
= $2760

Total amount collected from all the customers
= 6 u - 4140 + 2760
= 6 u - 1380

Glen collected a total of $80 more than what he had paid for.

6 u - 1380 = 4 u + 80
6 u - 4 u = 80 + 1380
2 u = 1460

1 u = 1460 ÷ 2 = 730

Number of customers who bought just one file
= 1 u - 690
= 730 - 690
= 40

Answer(s): 40