Andy bought some markers at $6 each. He later sold them to his customers at $8 each. For customers who bought three markers were given another marker for free. The customers bought either one or three markers. Andy gave away 130 free markers and collected a total of $380 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 4 markers each
Number	1 u	1 u - 520	520
Value	$6	$8	$8
Total Value	6 u	8 u - $4160	$3120

Since 130 markers were given free, there were 130 customers who bought three markers each.

Number of markers that each customer would get if they bought three markers each and got 1 marker free
= 3 + 1
= 4

Total number of markers for customers who each bought three markers 
= 4 x 130
= 520

Number of markers for customers who each bought 1 marker = 1 u - 520

Total amount collected from customers who bought 1 marker each
= 8 x (1 u - 520)
= 8 u - 4160

Total amount collected from customers who bought three markers each
= 130 x 3 x 8
= $3120

Total amount collected from all the customers
= 8 u - 4160 + 3120
= 8 u - 1040

Andy collected a total of $380 more than what he had paid for.

8 u - 1040 = 6 u + 380
8 u - 6 u = 380 + 1040
2 u = 1420

1 u = 1420 ÷ 2 = 710

Number of customers who bought just one marker
= 1 u - 520
= 710 - 520
= 190

Answer(s): 190