Answer: $ 18.75
Step-by-step 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
⇒
x
=
15
×
100
80
:
x
=
$
18.75
Method 2: form an equation and solve it
Let the original amount be
x
80
100
×
x
=
15
⇒
x
=
15
×
100
80
x
=
$
18.75
Method 3. Increase 15 in the ratio 100 : 80
15
×
100
80
x
=
$
18.75
Method 4: unitary method
$15 represents 80%
15
80
represents
1
%
15
80
×
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.