John bought some markers at $2 each. He later sold them to his customers at $5 each. For customers who bought three markers were given another marker for free. The customers bought either one or three markers. John gave away 150 free markers and collected a total of $1080 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 - 600	600
Value	$2	$5	$5
Total Value	2 u	5 u - $3000	$2250

Since 150 markers were given free, there were 150 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 150
= 600

Number of markers for customers who each bought 1 marker = 1 u - 600

Total amount collected from customers who bought 1 marker each
= 5 x (1 u - 600)
= 5 u - 3000

Total amount collected from customers who bought three markers each
= 150 x 3 x 5
= $2250

Total amount collected from all the customers
= 5 u - 3000 + 2250
= 5 u - 750

John collected a total of $1080 more than what he had paid for.

5 u - 750 = 2 u + 1080
5 u - 2 u = 1080 + 750
3 u = 1830

1 u = 1830 ÷ 3 = 610

Number of customers who bought just one marker
= 1 u - 600
= 610 - 600
= 10

Answer(s): 10