Asweater that originally cost $40 is on sale for 10 percent off. what is the amount of the discount?
$2
$4
$30
$36

X percent = x/100
10 percent = 10/100 = 0.10

10% of $40 = 0.10*40 = 4

You save $4

The old price is $40 and changes to $36 after the discount is applied. This is ignoring any taxes or other surcharges

Anyways I'm going a bit off tangent. The final answer is choice B) $4

The shelves will hold 270 books in total