You can swap the percentage and the amount: 16% of £25 is difficult to calculate in your head, but 25% of £16 is precisely the same amount.
So if you want to quickly work out 16% of £25 without a calculator, find 25% of £16 instead (ie divide £16 by 4). Simple!
Think about why this works.
Source: www.aperiodical.com
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