Sam bought some pens at $2 each. He later sold them to his customers at $5 each. For customers who bought two pens were given another pen for free. The customers bought either one or two pens. Sam gave away 170 free pens and collected a total of $1310 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 - 510	510
Value	$2	$5	$5
Total Value	2 u	5 u - $2550	$1700

Since 170 pens were given free, there were 170 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 170
= 510

Number of pens for customers who each bought 1 pen = 1 u - 510

Total amount collected from customers who bought 1 pen each
= 5 x (1 u - 510)
= 5 u - 2550

Total amount collected from customers who bought two pens each
= 170 x 2 x 5
= $1700

Total amount collected from all the customers
= 5 u - 2550 + 1700
= 5 u - 850

Sam collected a total of $1310 more than what he had paid for.

5 u - 850 = 2 u + 1310
5 u - 2 u = 1310 + 850
3 u = 2160

1 u = 2160 ÷ 3 = 720

Number of customers who bought just one pen
= 1 u - 510
= 720 - 510
= 210

Answer(s): 210