Eric bought some markers at $4 each. He later sold them to his customers at $8 each. For customers who bought four markers were given another marker for free. The customers bought either one or four markers. Eric gave away 110 free markers and collected a total of $1680 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 - 550	550
Value	$4	$8	$8
Total Value	4 u	8 u - $4400	$3520

Since 110 markers were given free, there were 110 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 110
= 550

Number of markers for customers who each bought 1 marker = 1 u - 550

Total amount collected from customers who bought 1 marker each
= 8 x (1 u - 550)
= 8 u - 4400

Total amount collected from customers who bought four markers each
= 110 x 4 x 8
= $3520

Total amount collected from all the customers
= 8 u - 4400 + 3520
= 8 u - 880

Eric collected a total of $1680 more than what he had paid for.

8 u - 880 = 4 u + 1680
8 u - 4 u = 1680 + 880
4 u = 2560

1 u = 2560 ÷ 4 = 640

Number of customers who bought just one marker
= 1 u - 550
= 640 - 550
= 90

Answer(s): 90