Lee bought some markers at $6 each. He later sold them to his customers at $9 each. For customers who bought three markers were given another marker for free. The customers bought either one or three markers. Lee gave away 110 free markers and collected a total of $870 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 - 440	440
Value	$6	$9	$9
Total Value	6 u	9 u - $3960	$2970

Since 110 markers were given free, there were 110 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 110
= 440

Number of markers for customers who each bought 1 marker = 1 u - 440

Total amount collected from customers who bought 1 marker each
= 9 x (1 u - 440)
= 9 u - 3960

Total amount collected from customers who bought three markers each
= 110 x 3 x 9
= $2970

Total amount collected from all the customers
= 9 u - 3960 + 2970
= 9 u - 990

Lee collected a total of $870 more than what he had paid for.

9 u - 990 = 6 u + 870
9 u - 6 u = 870 + 990
3 u = 1860

1 u = 1860 ÷ 3 = 620

Number of customers who bought just one marker
= 1 u - 440
= 620 - 440
= 180

Answer(s): 180