Carl bought some files at $5 each. He later sold them to his customers at $10 each. For customers who bought three files were given another file for free. The customers bought either one or three files. Carl gave away 120 free files and collected a total of $1350 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 4 files each
Number	1 u	1 u - 480	480
Value	$5	$10	$10
Total Value	5 u	10 u - $4800	$3600

Since 120 files were given free, there were 120 customers who bought three files each.

Number of files that each customer would get if they bought three files each and got 1 file free
= 3 + 1
= 4

Total number of files for customers who each bought three files 
= 4 x 120
= 480

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

Total amount collected from customers who bought 1 file each
= 10 x (1 u - 480)
= 10 u - 4800

Total amount collected from customers who bought three files each
= 120 x 3 x 10
= $3600

Total amount collected from all the customers
= 10 u - 4800 + 3600
= 10 u - 1200

Carl collected a total of $1350 more than what he had paid for.

10 u - 1200 = 5 u + 1350
10 u - 5 u = 1350 + 1200
5 u = 2550

1 u = 2550 ÷ 5 = 510

Number of customers who bought just one file
= 1 u - 480
= 510 - 480
= 30

Answer(s): 30