Flynn bought some markers at $5 each. He later sold them to his customers at $9 each. For customers who bought four markers were given another marker for free. The customers bought either one or four markers. Flynn gave away 120 free markers and collected a total of $1800 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 5 markers each
Number	1 u	1 u - 600	600
Value	$5	$9	$9
Total Value	5 u	9 u - $5400	$4320

Since 120 markers were given free, there were 120 customers who bought four markers each.

Number of markers that each customer would get if they bought four markers each and got 1 marker free
= 4 + 1
= 5

Total number of markers for customers who each bought four markers 
= 5 x 120
= 600

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

Total amount collected from customers who bought 1 marker each
= 9 x (1 u - 600)
= 9 u - 5400

Total amount collected from customers who bought four markers each
= 120 x 4 x 9
= $4320

Total amount collected from all the customers
= 9 u - 5400 + 4320
= 9 u - 1080

Flynn collected a total of $1800 more than what he had paid for.

9 u - 1080 = 5 u + 1800
9 u - 5 u = 1800 + 1080
4 u = 2880

1 u = 2880 ÷ 4 = 720

Number of customers who bought just one marker
= 1 u - 600
= 720 - 600
= 120

Answer(s): 120