Henry bought some files at $6 each. He later sold them to his customers at $8 each. For customers who bought four files were given another file for free. The customers bought either one or four files. Henry gave away 140 free files and collected a total of $460 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 - 700	700
Value	$6	$8	$8
Total Value	6 u	8 u - $5600	$4480

Since 140 files were given free, there were 140 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 140
= 700

Number of files for customers who each bought 1 file = 1 u - 700

Total amount collected from customers who bought 1 file each
= 8 x (1 u - 700)
= 8 u - 5600

Total amount collected from customers who bought four files each
= 140 x 4 x 8
= $4480

Total amount collected from all the customers
= 8 u - 5600 + 4480
= 8 u - 1120

Henry collected a total of $460 more than what he had paid for.

8 u - 1120 = 6 u + 460
8 u - 6 u = 460 + 1120
2 u = 1580

1 u = 1580 ÷ 2 = 790

Number of customers who bought just one file
= 1 u - 700
= 790 - 700
= 90

Answer(s): 90