David bought some staplers at $5 each. He later sold them to his customers at $7 each. For customers who bought two staplers were given another stapler for free. The customers bought either one or two staplers. David gave away 140 free staplers and collected a total of $540 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 3 staplers each
Number	1 u	1 u - 420	420
Value	$5	$7	$7
Total Value	5 u	7 u - $2940	$1960

Since 140 staplers were given free, there were 140 customers who bought two staplers each.

Number of staplers that each customer would get if they bought two staplers each and got 1 stapler free
= 2 + 1
= 3

Total number of staplers for customers who each bought two staplers 
= 3 x 140
= 420

Number of staplers for customers who each bought 1 stapler = 1 u - 420

Total amount collected from customers who bought 1 stapler each
= 7 x (1 u - 420)
= 7 u - 2940

Total amount collected from customers who bought two staplers each
= 140 x 2 x 7
= $1960

Total amount collected from all the customers
= 7 u - 2940 + 1960
= 7 u - 980

David collected a total of $540 more than what he had paid for.

7 u - 980 = 5 u + 540
7 u - 5 u = 540 + 980
2 u = 1520

1 u = 1520 ÷ 2 = 760

Number of customers who bought just one stapler
= 1 u - 420
= 760 - 420
= 340

Answer(s): 340