Question

A store owner paid $15 for a book. She marked up the price of the book by 40% to determine its selling price. What is the selling price of th book?

Answer 1

the book is 15 dollars,
15/100=0.15 this number represents 1% of the price
the store owner marked up the price by 40%
40*0.15=6
now the book costs
15+6=21 dollars

Answer 2

If you'd like to do mental calculations,
$15 ÷ 100 × 140 = $15 ÷ 10 × 14
= $1.50 × 14
= $1.50 × 10 + $1.50 × 4
= $15 + $6
= $21

Another way:
$15 × 140/100 = $15 × 14/10
= $15 × 7/5 <simplify fraction as much as possible>
= $15/1 × 7/5
<Multiply numerator with numerator and denominator with denominator>
= $(15 × 7)/5
= $105/5
= $21

If you have a calculator, type:
15 × 140%
It should give you the same answer as well.

Hope this helps! :)