X as a percent of Y = X ÷ Y × 100. 30 is 15% of 200.
Change = (new − old) ÷ old × 100. From 200 to 230 is a 15% increase.
A percent change always divides by the starting value, which is why a 50% fall followed by a 50% rise does not get you back where you started: 100 becomes 50, and 50 becomes 75.
Percentage points are different from percent: a rate going from 4% to 5% rises by one percentage point but by 25%.
Common questions
How do I calculate a percentage in my head?
Find 10% by moving the decimal point one place left, then scale: 30% of 70 is 3 × 7 = 21. X% of Y also equals Y% of X, so 8% of 50 is 50% of 8 = 4.
How do I reverse a percentage increase?
Divide by 1 plus the rate: a price of 115 after a 15% rise was 115 ÷ 1.15 = 100.