Unitary method for calculating percentage 'of'

There is sometimes an easier way to calculate percentages. It is called the 'unitary method'. In this case, first work out 10% by dividing the number by ten in your head, then work in multiples or fractions of ten.

Example 1

Find 35% of 20.

Solution

\begin{align}10\%\ \text{of}\ 20&=2\\\\ \text{So}\ 5\%\ \text{of}\ 20\ \text{is half of}\ 10\% &=1\\\\ \text{So}\ 35\%\ \text{of}\ 20 &=7 × 5\%\ \text{of}\ 20\\\\ &=7×1\\\\ &=7\end{align}