Ahmad bought some files at $6 each. He later sold them to his customers at $10 each. For customers who bought two files were given another file for free. The customers bought either one or two files. Ahmad gave away 120 free files and collected a total of $1680 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 - 360	360
Value	$6	$10	$10
Total Value	6 u	10 u - $3600	$2400

Since 120 files were given free, there were 120 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 120
= 360

Number of files for customers who each bought 1 file = 1 u - 360

Total amount collected from customers who bought 1 file each
= 10 x (1 u - 360)
= 10 u - 3600

Total amount collected from customers who bought two files each
= 120 x 2 x 10
= $2400

Total amount collected from all the customers
= 10 u - 3600 + 2400
= 10 u - 1200

Ahmad collected a total of $1680 more than what he had paid for.

10 u - 1200 = 6 u + 1680
10 u - 6 u = 1680 + 1200
4 u = 2880

1 u = 2880 ÷ 4 = 720

Number of customers who bought just one file
= 1 u - 360
= 720 - 360
= 360

Answer(s): 360