Question

Marco purchased a large box of comic books for $300. He gave 15 of the comic books to his brother and then sold the rest on the internet for $330, making a profit of $1.50 on each one. How many comic books were in the box? What was the original price of each comics book (assuming they all cost the same amount)?

Answer 1

number of comic books he bought --- x
cost price per comic = 300/x
number sold = x-15
selling price of each comic = 330/(x-15)

330/(x-15) - 300/x = 1.50 = 3/2
multiply each term by 2x(x-15)
660x - 600(x-15) = 3x(x-15)
660x - 600x + 9000 = 3x^2 - 45x
3x^2 - 105x - 9000 = 0
x^2 - 35x - 3000 = 0
(x + 40)(x - 75) = 0
x = 75 or a negative, which we reject

He bought 75 comic books at $4.00 each

check: 75*4 = 300 , check!
60*5.5 = 330, check!