Lee bought some pens at $2 each. He later sold them to his customers at $4 each. For customers who bought two pens were given another pen for free. The customers bought either one or two pens. Lee gave away 140 free pens and collected a total of $860 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 - 420	420
Value	$2	$4	$4
Total Value	2 u	4 u - $1680	$1120

Since 140 pens were given free, there were 140 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 140
= 420

Number of pens for customers who each bought 1 pen = 1 u - 420

Total amount collected from customers who bought 1 pen each
= 4 x (1 u - 420)
= 4 u - 1680

Total amount collected from customers who bought two pens each
= 140 x 2 x 4
= $1120

Total amount collected from all the customers
= 4 u - 1680 + 1120
= 4 u - 560

Lee collected a total of $860 more than what he had paid for.

4 u - 560 = 2 u + 860
4 u - 2 u = 860 + 560
2 u = 1420

1 u = 1420 ÷ 2 = 710

Number of customers who bought just one pen
= 1 u - 420
= 710 - 420
= 290

Answer(s): 290