Albert bought some staplers at $4 each. He later sold them to his customers at $9 each. For customers who bought three staplers were given another stapler for free. The customers bought either one or three staplers. Albert gave away 120 free staplers and collected a total of $2670 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 4 staplers each
Number	1 u	1 u - 480	480
Value	$4	$9	$9
Total Value	4 u	9 u - $4320	$3240

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

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

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

Number of staplers for customers who each bought 1 stapler = 1 u - 480

Total amount collected from customers who bought 1 stapler each
= 9 x (1 u - 480)
= 9 u - 4320

Total amount collected from customers who bought three staplers each
= 120 x 3 x 9
= $3240

Total amount collected from all the customers
= 9 u - 4320 + 3240
= 9 u - 1080

Albert collected a total of $2670 more than what he had paid for.

9 u - 1080 = 4 u + 2670
9 u - 4 u = 2670 + 1080
5 u = 3750

1 u = 3750 ÷ 5 = 750

Number of customers who bought just one stapler
= 1 u - 480
= 750 - 480
= 270

Answer(s): 270