Simplify calculating percentage

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.