Tom is selling 1200 story books. He sold 75% of them at $10 each, 60% of the remainder at a discount of 40% and the rest at cost price. He made a profit of $2031. Find the cost price of a story book.

Price	$10	$6	Cost Price
Percent	75%	25%
		15%	10%
Number	900	180	Not needed
Amount collected	$9000	$1080	Not needed

Number of story books sold at $10
= 75% x 1200
= ⁷⁵₁₀₀ x 1200
= 900

Price after 40% discount
= 60% x 10
= ⁶⁰₁₀₀ x 10
= $6

Remaining story books
= 1200 - 900
= 300

Number of story books sold at $6
= 60% x 300
= ⁶⁰₁₀₀ x 300
= 180

Number of story books sold at $10 or 40% discount
= 900 + 180
= 1080

Total amount collected from the sale of 900 story books
= 900 x 10
= $9000

Total amount collected from the sale of the other 180 story books
= 180 x 6
= $1080

Total amount collected from the sale of story books sold at $10 or 40% discount
= 9000 + 1080
= $10080

Cost price of 1080 story books
= 10080 - 2031
= $4320

Cost price of 1 story book
= 4320 ÷ 1080
= $4

Answer(s): $4