Will bought some pens at $5 each. He later sold them to his customers at $10 each. For customers who bought two pens were given another pen for free. The customers bought either one or two pens. Will gave away 180 free pens and collected a total of $1000 more than what he had paid for. How many customers bought just one pen each?

	Pens sold	Pens given to customers who got 1 pen each	Pens given to customers who got 3 pens each
Number	1 u	1 u - 540	540
Value	$5	$10	$10
Total Value	5 u	10 u - $5400	$3600

Since 180 pens were given free, there were 180 customers who bought two pens each.

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

Total number of pens for customers who each bought two pens 
= 3 x 180
= 540

Number of pens for customers who each bought 1 pen = 1 u - 540

Total amount collected from customers who bought 1 pen each
= 10 x (1 u - 540)
= 10 u - 5400

Total amount collected from customers who bought two pens each
= 180 x 2 x 10
= $3600

Total amount collected from all the customers
= 10 u - 5400 + 3600
= 10 u - 1800

Will collected a total of $1000 more than what he had paid for.

10 u - 1800 = 5 u + 1000
10 u - 5 u = 1000 + 1800
5 u = 2800

1 u = 2800 ÷ 5 = 560

Number of customers who bought just one pen
= 1 u - 540
= 560 - 540
= 20

Answer(s): 20