Per cent means 'per hundred'. x% of a quantity is x/100 of it.
1Basics
x% of y = xy/100
a is (a/b × 100)% of b
x% = x/100 as a fraction, e.g. 12.5% = 1/8
2Successive changes
An increase of a% followed by b%: net = a + b + ab/100 (use negative values for decreases)
Price up by r%: to keep spending the same, reduce consumption by r/(100 + r) × 100%
If A is r% more than B, B is r/(100 + r) × 100% less than A
3Population and depreciation
After n years at r% growth: P(1 + r/100)ⁿ
After n years at r% depreciation: P(1 − r/100)ⁿ