In general, when calculating percentages, the percentages must first be converted to fractions or decimals. A percentage value can be calculated from a number by multiplying the base value by a percentage factor.

Example 1

How much is [[$ 20 \: \% $]]​ from the number [[$ 150 $]]​?

Example 2

A pair of jeans shrinks by [[$ 3 \: \% $]]​ the first time they are washed. How much shorter will the jeans be after the first wash if their original length was [[$ 98 \: \text {cm} $]]​?

Solution:

[[$ \displaystyle\frac {3} {100} \cdot 98 \: \text {cm} = 0,03 \cdot 98 \: \text {cm} ≈ 2,9 \: \text {cm} $]]​

Answer: The jeans will shrink by [[$ 2,9\: \text {cm} $]]​.