Bryan bought some files at $2 each. He later sold them to his customers at $7 each. For customers who bought four files were given another file for free. The customers bought either one or four files. Bryan gave away 130 free files and collected a total of $2990 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 5 files each
Number	1 u	1 u - 650	650
Value	$2	$7	$7
Total Value	2 u	7 u - $4550	$3640

Since 130 files were given free, there were 130 customers who bought four files each.

Number of files that each customer would get if they bought four files each and got 1 file free
= 4 + 1
= 5

Total number of files for customers who each bought four files 
= 5 x 130
= 650

Number of files for customers who each bought 1 file = 1 u - 650

Total amount collected from customers who bought 1 file each
= 7 x (1 u - 650)
= 7 u - 4550

Total amount collected from customers who bought four files each
= 130 x 4 x 7
= $3640

Total amount collected from all the customers
= 7 u - 4550 + 3640
= 7 u - 910

Bryan collected a total of $2990 more than what he had paid for.

7 u - 910 = 2 u + 2990
7 u - 2 u = 2990 + 910
5 u = 3900

1 u = 3900 ÷ 5 = 780

Number of customers who bought just one file
= 1 u - 650
= 780 - 650
= 130

Answer(s): 130