Ken bought some markers at $6 each. He later sold them to his customers at $8 each. For customers who bought two markers were given another marker for free. The customers bought either one or two markers. Ken gave away 140 free markers and collected a total of $480 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 - 420	420
Value	$6	$8	$8
Total Value	6 u	8 u - $3360	$2240

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

Number of markers for customers who each bought 1 marker = 1 u - 420

Total amount collected from customers who bought 1 marker each
= 8 x (1 u - 420)
= 8 u - 3360

Total amount collected from customers who bought two markers each
= 140 x 2 x 8
= $2240

Total amount collected from all the customers
= 8 u - 3360 + 2240
= 8 u - 1120

Ken collected a total of $480 more than what he had paid for.

8 u - 1120 = 6 u + 480
8 u - 6 u = 480 + 1120
2 u = 1600

1 u = 1600 ÷ 2 = 800

Number of customers who bought just one marker
= 1 u - 420
= 800 - 420
= 380

Answer(s): 380