Type the answer and press Enter. All answers are whole numbers. How many can you solve before time runs out?
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Mental percentage tricks
Nearly every percentage can be built from a few anchors. 10% is just the decimal shifted (10% of 340 = 34); 5% is half of that; 1% is two shifts. 25% is a quarter, 75% is three quarters. And the reversal trick often saves you: x% of y always equals y% of x, so 4% of 50 becomes 50% of 4 = 2. For “A is what % of B”, divide and scale: 30 of 120 → 30/120 = 1/4 = 25%. The quiz rotates all three question types so the anchors become automatic.
Building any percentage from three anchors
Almost every percentage question can be assembled from three values you can find instantly. Ten percent is the decimal point moved one place left, so 10% of 340 is 34. One percent moves it two places, giving 3.4. Five percent is half of ten, giving 17. From those three, 15% is 34 plus 17, 20% is 34 doubled, and 35% is 34 plus 34 plus 17.
Quarters are worth adding to the set. Twenty-five percent is a quarter and seventy-five percent is three quarters, both of which are usually faster found by dividing than by building from tens.
The reversal trick
x% of y always equals y% of x. This one identity turns a large number of awkward questions into trivial ones. Four percent of 50 looks like work; fifty percent of 4 is 2. Eighteen percent of 50 is 50 percent of 18, which is 9. Sixteen percent of 25 is 25 percent of 16, which is 4.
It works because a percentage is a multiplication, and multiplication does not care about order. Once you see it that way you start noticing which of the two numbers is the easy one to take a percentage of, and taking that one instead.
The three question types here
What is x% of y? Build it from ten, one and five percent as above.
A is what percent of B? Divide and scale. 30 out of 120 is a quarter, so 25 percent.
Increase or decrease by x%. A 20 percent rise is multiplying by 1.2; a 20 percent fall is multiplying by 0.8.
A rise of 20 percent followed by a fall of 20 percent does not return you to the start — it leaves you at 96 percent of it.
Every answer in this quiz is a whole number, so a fractional result means the method went wrong somewhere.
What's the fastest way to compute a percentage in my head?
Anchor on 10% (shift the decimal) and build from there: 5% is half of 10%, 20% is double, 1% is two shifts. For 15% of 60: 10% is 6, 5% is 3, total 9. With the anchors automatic, most everyday percentages take a second or two.
What is the percentage reversal trick?
x% of y always equals y% of x, because both are x·y/100. So an awkward 4% of 75 becomes an easy 75% of 4 = 3, and 16% of 25 becomes 25% of 16 = 4. Whenever one direction feels hard, flip it.
Why do the questions only use 5, 10, 20, 25, 50 and 75 percent?
Those are the anchors real life uses most — tips, discounts, sales tax estimates — and they guarantee whole-number answers so you can focus on speed. Once they're automatic, odd percentages become combinations of them (e.g. 35% = 25% + 10%).
What's the difference between a percentage and a percentage point?
Percentage points measure the plain gap between two percentages; percent measures relative change. If a rate rises from 10% to 15%, that's 5 percentage points — but a 50% increase, since 5 is half of 10. News headlines mix these up constantly, which is why the distinction is worth knowing.
How do I work out what percent one number is of another?
Divide the part by the whole, then scale to 100: 30 out of 120 is 30/120 = 1/4 = 25%. In your head, look for a simplifying fraction first — halves, quarters, fifths and tenths cover most cases. This quiz includes that question type precisely because it trips people up more than the other direction.
Why don't a 20% increase and a 20% decrease cancel out?
Because each percentage applies to a different base. Raise 100 by 20% and you get 120; cut 120 by 20% and you lose 24, landing at 96. This asymmetry matters in real life — a price that drops 50% must then rise 100% to recover — and it's a favourite trap in aptitude tests.