George bought some staplers at $4 each. He later sold them to his customers at $7 each. For customers who bought four staplers were given another stapler for free. The customers bought either one or four staplers. George gave away 120 free staplers and collected a total of $1020 more than what he had paid for. How many customers bought just one stapler each?

	Staplers sold	Staplers given to customers who got 1 stapler each	Staplers given to customers who got 5 staplers each
Number	1 u	1 u - 600	600
Value	$4	$7	$7
Total Value	4 u	7 u - $4200	$3360

Since 120 staplers were given free, there were 120 customers who bought four staplers each.

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

Total number of staplers for customers who each bought four staplers 
= 5 x 120
= 600

Number of staplers for customers who each bought 1 stapler = 1 u - 600

Total amount collected from customers who bought 1 stapler each
= 7 x (1 u - 600)
= 7 u - 4200

Total amount collected from customers who bought four staplers each
= 120 x 4 x 7
= $3360

Total amount collected from all the customers
= 7 u - 4200 + 3360
= 7 u - 840

George collected a total of $1020 more than what he had paid for.

7 u - 840 = 4 u + 1020
7 u - 4 u = 1020 + 840
3 u = 1860

1 u = 1860 ÷ 3 = 620

Number of customers who bought just one stapler
= 1 u - 600
= 620 - 600
= 20

Answer(s): 20