Calculate percentages without getting tripped up.
A 10-minute lesson: what a percentage is, mental math with 10% and 1%, its 3 pieces, typical traps, and a real-ticket challenge.
What a percentage is, how to calculate it in your head, and how to avoid its usual traps.
The case
30% + 20% isn't 50%. And 30% of 200 isn't guesswork.
In 10 minutes you'll know what a percentage means, turn it into mental math, and spot the two classic traps.
What you will learn
- Explain what a percentage means relative to its base
- Calculate 10%, 50%, and 25% in your head
- Spot wrongly added successive discounts
Parts of a hundred
A percentage is parts of each hundred. See it that way and the math is one multiplication — and the traps give themselves away.
Base
The total you calculate on: the hundred.
Rate
How many parts of a hundred you take.
Part
The result: base times rate over a hundred.
Trick
10% is dividing by 10; 1% by 100.
Frequently asked questions
What does 30% mean?
30 parts of each 100 of a specific base: 30% of 200 is 60, but 30% of 50 is 15.
How do I calculate 10% in my head?
By dividing by 10: move the decimal point one place. 80 → 8.
Test yourself: Lightning test: clear math
What does 30% mean?
Correct answer: 30 parts of each 100.
The % is always relative to a base: 30% of 200 is 60, but 30% of 50 is 15.
What is 10% of 80?
Correct answer: 8: move the decimal point one place.
10% is dividing by 10: 80 → 8. It's the base trick of mental math.
A shop applies −30% then −20%. Is that a 50% discount?
Correct answer: No: the second runs on what was left, giving 44%.
100 → 70 → 56: you pay 56, you save 44. Adding percentages is the classic trap.
The pocket rule — Every percentage is base × rate / 100. And two discounts in a row chain together — they never add up.
What you take away
Calculate basic percentages and spot their two traps.