Sometimes a difficult percentage calculation can be made easy simply by swapping the two terms.

For example: In your head, you may find it difficult to compute 18% of 25. Instead, try solving for 25% of 18.

25% is just half of half of any number. Half of 18 is 9, and half of 9 is 4.5.

Since 25% of 18 is 4.5, it is also true that 18% of 25 is 4.5.

This is something my teachers never taught me in school, and it would have helped a lot in everyday life -- A% of B = B% of A.

Like multiplication, percentages conform to the commutative principle, which says the order of the terms does not matter. (Just as \(2 \times 3 = 3 \times 2\), it is also true that 40% of 5 = 5% of 40).

Submitted by former curiousmath.com member "deud".