Paul bought some markers at $5 each. He later sold them to his customers at $7 each. For customers who bought four markers were given another marker for free. The customers bought either one or four markers. Paul gave away 100 free markers and collected a total of $340 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 - 500	500
Value	$5	$7	$7
Total Value	5 u	7 u - $3500	$2800

Since 100 markers were given free, there were 100 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 100
= 500

Number of markers for customers who each bought 1 marker = 1 u - 500

Total amount collected from customers who bought 1 marker each
= 7 x (1 u - 500)
= 7 u - 3500

Total amount collected from customers who bought four markers each
= 100 x 4 x 7
= $2800

Total amount collected from all the customers
= 7 u - 3500 + 2800
= 7 u - 700

Paul collected a total of $340 more than what he had paid for.

7 u - 700 = 5 u + 340
7 u - 5 u = 340 + 700
2 u = 1040

1 u = 1040 ÷ 2 = 520

Number of customers who bought just one marker
= 1 u - 500
= 520 - 500
= 20

Answer(s): 20