Question 1

How do I calculate X% of Y?

Accepted Answer

To find X% of Y, multiply Y by X and divide by 100. Formula: Result = (X / 100) × Y. Examples: 20% of 150 = (20/100) × 150 = 30; 15% of £80 (tip calculation) = (15/100) × 80 = £12; 7.5% of 2400 (VAT example) = (7.5/100) × 2400 = 180; 0.5% of 10,000 (interest rate) = 50. You can also convert: 20% = 0.2 as a decimal, so 0.2 × 150 = 30. The calculator handles all of these instantly.

Question 2

How do I calculate percentage change?

Accepted Answer

Percentage change formula: % Change = ((New Value − Old Value) / Old Value) × 100. A positive result is an increase; a negative result is a decrease. Examples: Old price £80, new price £100 → (100-80)/80 × 100 = +25% increase. Old price £100, new price £80 → (80-100)/100 × 100 = -20% decrease. Note: a 25% increase and a 20% decrease are not symmetrical — that is normal. Traffic last month 1,200, this month 1,500 → (1500-1200)/1200 × 100 = +25% growth.

Question 3

What is the difference between percentage points and percentage change?

Accepted Answer

This is one of the most common sources of confusion in statistics. A percentage point is an arithmetic difference between two percentages. A percentage change is the relative change. Example: interest rate rises from 2% to 3%. That is an increase of 1 percentage point (3 − 2 = 1 pp). But the percentage change is 50% ((3-2)/2 × 100 = 50%). Both statements are correct in different contexts. Politicians and journalists often confuse these — 'interest rates increased by 50%' (relative) vs. 'increased by 1 percentage point' (absolute) mean very different things.

Question 4

How do I add or subtract a percentage from a number?

Accepted Answer

To add X% to a number N: Result = N × (1 + X/100). To subtract X% from N: Result = N × (1 − X/100). Examples: Add 20% VAT to £150: 150 × 1.20 = £180. Subtract 20% VAT from £180 (reverse): 180 / 1.20 = £150 (not 180 × 0.80 = £144, which would be a 20% discount, not VAT removal). Apply 10% discount to £200: 200 × 0.90 = £180. This is the mode to use for VAT calculations, discount calculations and salary percentage increases.

Question	Formula	Example
What is X% of Y?	Y × (X / 100)	20% of 150 = 30
X is what % of Y?	(X / Y) × 100	30 is what % of 150? = 20%
% change from X to Y	((Y − X) / X) × 100	80 → 100 = +25%; 100 → 80 = −20%
Add X% to Y (e.g. VAT)	Y × (1 + X/100)	Add 20% to £150 = £180
Subtract X% from Y	Y × (1 − X/100)	10% off £200 = £180

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