#### Explanation:

The $\textcolor{b l u e}{\text{original price}}$ is 100%

#### Explanation:

Do not be tempted to increase $15 by 20% thinking that will make the original 100%. 20% of the original amount and 20% of the reduced amount are different quantities. There are several different approaches: Method 1: direct proportion Compare the values with their corresponding percentages. 15/80 = x/100 " "rArr " " x = (15xx100)/80:" " x =$18.75

Method 2: form an equation and solve it

Let the original amount be $x$

80/100 xx x = 15" " rArr " "x = 15 xx 100/80 " "x = $18.75 Method 3. Increase 15 in the ratio 100 : 80 15 xx100/80 " " x =$18.75

Method 4: unitary method

$15 represents 80% 15/80 " represents " 1% 15/80 xx 100 " represents" 100%" "$18.75

Notice that in each case, although the approach and reasoning were different, the calculations and the answer were the same.

The important thing is to recognise that this is a reverse percentage calculation, because the original amount is not known.