Charlie bought some pencils at $3 each. He later sold them to his customers at $5 each. For customers who bought three pencils were given another pencil for free. The customers bought either one or three pencils. Charlie gave away 140 free pencils and collected a total of $780 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 4 pencils each
Number	1 u	1 u - 560	560
Value	$3	$5	$5
Total Value	3 u	5 u - $2800	$2100

Since 140 pencils were given free, there were 140 customers who bought three pencils each.

Number of pencils that each customer would get if they bought three pencils each and got 1 pencil free
= 3 + 1
= 4

Total number of pencils for customers who each bought three pencils 
= 4 x 140
= 560

Number of pencils for customers who each bought 1 pencil = 1 u - 560

Total amount collected from customers who bought 1 pencil each
= 5 x (1 u - 560)
= 5 u - 2800

Total amount collected from customers who bought three pencils each
= 140 x 3 x 5
= $2100

Total amount collected from all the customers
= 5 u - 2800 + 2100
= 5 u - 700

Charlie collected a total of $780 more than what he had paid for.

5 u - 700 = 3 u + 780
5 u - 3 u = 780 + 700
2 u = 1480

1 u = 1480 ÷ 2 = 740

Number of customers who bought just one pencil
= 1 u - 560
= 740 - 560
= 180

Answer(s): 180