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90% of 20 is 18. That’s the whole answer, and you can get there in under five seconds once you know one little trick. Stick around and you’ll never need a calculator for this again.
The short answer: 90 percent of 20 equals 18
If you landed here asking “what is 90% of 20?”, here it is in plain numbers:
- 90% of 20 = 18
- The leftover 10% = 2
- 18 + 2 = 20, the whole you started with
So 90 percent of 20 is 18, which means you’re keeping nearly all of the 20 and shaving off just 2.
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METHOD 1 The Classic Percentage Formula
To calculate a percentage of a number, convert the percentage into a decimal and multiply.
Let's find 90% of 20:
- Write the percentage as a fraction: 90 ÷ 100
- Convert it into a decimal: 90 ÷ 100 = 0.90
- Multiply by 20: 20 × 0.90 = 18
METHOD 2 The Subtract-10% Shortcut
Since 90% is just 10% less than 100%, find 10% of 20 and subtract it from 20.
- Find 10% of 20: 20 ÷ 10 = 2
- Subtract 2 from the whole number: 20 − 2 = 18
This shortcut works because 90% = 100% − 10%.
METHOD 3 The Fraction Route
Percentages can also be written as fractions. The percentage 90% is equivalent to 9/10.
- Divide 20 by 10: 20 ÷ 10 = 2
- Multiply the result by 9: 2 × 9 = 18
Quick Reference: Percentages of 20
Use this handy table to compare common percentages and their values.
| Percentage | Calculation | Result |
|---|---|---|
| 10% of 20 | 20 × 0.10 | 2 |
| 25% of 20 | 20 × 0.25 | 5 |
| 50% of 20 | 20 × 0.50 | 10 |
| 75% of 20 | 20 × 0.75 | 15 |
| 90% of 20 | 20 × 0.90 | 18 |
| 100% of 20 | 20 × 1.00 | 20 |
Where does 90% of 20 show up in real life?
It might look like a textbook question, but 90% of 20 pops up in everyday situations:
- Shopping: A $20 item with a 10% discount costs you 90% of the price, which is $18.
- School: Scoring 18 out of 20 on a quiz is exactly 90%, a solid A in most grading systems.
- Fitness: Finishing 18 of 20 planned workouts in a month means you hit 90% of your goal.
- Work: If a team of 20 people has 90% attendance, 18 are in the room.
- Battery life: A 20-hour battery at 90% charge gives you about 18 hours of use.
Notice the pattern? Whenever you see a 10% reduction, you’re really calculating 90 percent of the original.
Recommended Reading: How to Write 41000 in Words? | Forty-One Thousand
Common mistakes to avoid
- Mixing up the part and the whole. 90% of 20 is 18, not 20% of 90 (which also happens to be 18, a neat quirk explained below).
- Forgetting to convert. Multiplying 20 × 90 gives 1,800. You must divide by 100 first, or use 0.90.
- Subtracting the wrong amount. Taking 10 off instead of 10% off gives 10, not 18.
Recommended Reading: Is 0.75 the Same as 3/4? | Decimal to Fraction
A fun math quirk
Here’s something that surprises people: 90% of 20 equals 20% of 90. Both are 18. In fact, x% of y always equals y% of x, because the formula is just multiplication, and multiplication doesn’t care about order. If one version of a percentage problem feels hard, flip it. 4% of 75 sounds tricky, but 75% of 4 is just 3.
Conclusion
So, what is 90% of 20? It’s 18. Remember the three routes: multiply by 0.90, subtract 10% from the whole, or take nine-tenths. Pick whichever feels most natural, and the next time someone asks about 90% of 20, you’ll have the answer before they finish the sentence.
Bonus challenge: What is 90% of 200? What about 90% of 2,000? (Answers: 180 and 1,800. Just add zeros!)
You can opt for our Advanced Math or Vedic Math+Mental Math courses. Our Math Quiz for grades 3rd, 4th, 5th, and 6th helps in further exciting and engaging in mathematics with hands-on lessons.
