Warren bought some pencils at $4 each. He later sold them to his customers at $8 each. For customers who bought two pencils were given another pencil for free. The customers bought either one or two pencils. Warren gave away 210 free pencils and collected a total of $1200 more than what he had paid for. How many customers bought just one pencil each?

	Pencils sold	Pencils given to customers who got 1 pencil each	Pencils given to customers who got 3 pencils each
Number	1 u	1 u - 630	630
Value	$4	$8	$8
Total Value	4 u	8 u - $5040	$3360

Since 210 pencils were given free, there were 210 customers who bought two pencils each.

Number of pencils that each customer would get if they bought two pencils each and got 1 pencil free
= 2 + 1
= 3

Total number of pencils for customers who each bought two pencils 
= 3 x 210
= 630

Number of pencils for customers who each bought 1 pencil = 1 u - 630

Total amount collected from customers who bought 1 pencil each
= 8 x (1 u - 630)
= 8 u - 5040

Total amount collected from customers who bought two pencils each
= 210 x 2 x 8
= $3360

Total amount collected from all the customers
= 8 u - 5040 + 3360
= 8 u - 1680

Warren collected a total of $1200 more than what he had paid for.

8 u - 1680 = 4 u + 1200
8 u - 4 u = 1200 + 1680
4 u = 2880

1 u = 2880 ÷ 4 = 720

Number of customers who bought just one pencil
= 1 u - 630
= 720 - 630
= 90

Answer(s): 90