Pierre bought some files at $4 each. He later sold them to his customers at $8 each. For customers who bought two files were given another file for free. The customers bought either one or two files. Pierre gave away 190 free files and collected a total of $840 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 - 570	570
Value	$4	$8	$8
Total Value	4 u	8 u - $4560	$3040

Since 190 files were given free, there were 190 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 190
= 570

Number of files for customers who each bought 1 file = 1 u - 570

Total amount collected from customers who bought 1 file each
= 8 x (1 u - 570)
= 8 u - 4560

Total amount collected from customers who bought two files each
= 190 x 2 x 8
= $3040

Total amount collected from all the customers
= 8 u - 4560 + 3040
= 8 u - 1520

Pierre collected a total of $840 more than what he had paid for.

8 u - 1520 = 4 u + 840
8 u - 4 u = 840 + 1520
4 u = 2360

1 u = 2360 ÷ 4 = 590

Number of customers who bought just one file
= 1 u - 570
= 590 - 570
= 20

Answer(s): 20