Billy bought some markers at $5 each. He later sold them to his customers at $7 each. For customers who bought two markers were given another marker for free. The customers bought either one or two markers. Billy gave away 210 free markers and collected a total of $130 more than what he had paid for. How many customers bought just one marker each?

	Markers sold	Markers given to customers who got 1 marker each	Markers given to customers who got 3 markers each
Number	1 u	1 u - 630	630
Value	$5	$7	$7
Total Value	5 u	7 u - $4410	$2940

Since 210 markers were given free, there were 210 customers who bought two markers each.

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

Total number of markers for customers who each bought two markers 
= 3 x 210
= 630

Number of markers for customers who each bought 1 marker = 1 u - 630

Total amount collected from customers who bought 1 marker each
= 7 x (1 u - 630)
= 7 u - 4410

Total amount collected from customers who bought two markers each
= 210 x 2 x 7
= $2940

Total amount collected from all the customers
= 7 u - 4410 + 2940
= 7 u - 1470

Billy collected a total of $130 more than what he had paid for.

7 u - 1470 = 5 u + 130
7 u - 5 u = 130 + 1470
2 u = 1600

1 u = 1600 ÷ 2 = 800

Number of customers who bought just one marker
= 1 u - 630
= 800 - 630
= 170

Answer(s): 170