Fractions, Decimals and Percents: Conversions to Know
No calculator on the ASVAB means a few fraction, decimal and percent conversions save real time. The table to learn, three tricks and the percent traps.
There's no calculator on the ASVAB — on any version. That makes one small skill unusually valuable: moving between fractions, decimals and percents without thinking. A problem that says "37.5%" is really saying "3/8," and a problem that says "1/6" is really saying "about 16.7%." Knowing which form makes the arithmetic easy is half the work.
The conversions to know by heart
| Fraction | Decimal | Percent |
|---|---|---|
| 1/2 | 0.5 | 50% |
| 1/3 | 0.333… | 33⅓% |
| 2/3 | 0.666… | 66⅔% |
| 1/4 | 0.25 | 25% |
| 3/4 | 0.75 | 75% |
| 1/5 | 0.2 | 20% |
| 1/6 | 0.1666… | 16⅔% |
| 5/6 | 0.8333… | 83⅓% |
| 1/8 | 0.125 | 12.5% |
| 3/8 | 0.375 | 37.5% |
| 5/8 | 0.625 | 62.5% |
| 7/8 | 0.875 | 87.5% |
| 1/10 | 0.1 | 10% |
| 1/20 | 0.05 | 5% |
You don't need to memorize every row as a separate fact. Learn the "unit" fractions — 1/3, 1/4, 1/5, 1/6, 1/8 — and multiply: 3/8 is three times 12.5%, so 37.5%.
How to convert anything
- Fraction → decimal: divide the top by the bottom. 3/8 = 3 ÷ 8 = 0.375.
- Decimal → percent: move the decimal point two places right. 0.375 → 37.5%.
- Percent → decimal: move it two places left. 12.5% → 0.125.
- Decimal → fraction: write it over 10, 100 or 1,000 and simplify. 0.75 = 75/100 = 3/4.
Three tricks that save time
1. Build percents from 10% and 5%
To find 15% of 80: 10% of 80 is 8, and 5% is half of that, 4. So 15% is 8 + 4 = 12. The same building blocks give you 20%, 25%, 30% and 35% almost instantly.
2. Flip the percent
x% of y = y% of x. That sounds abstract, but it's a huge shortcut: 8% of 50 is the same as 50% of 8, which is 4. When one direction is awkward, try the other.
3. Switch to the fraction when it's cleaner
37.5% of 64 looks hard. 3/8 of 64 is easy: 64 ÷ 8 = 8, times 3 = 24.
The percent traps
Percent change goes from the starting value
From 40 to 50 is a 25% increase (10 ÷ 40). From 50 back to 40 is a 20% decrease (10 ÷ 50). Same 10-point difference, different percents, because each change is measured from where you start.
Two discounts don't add up
20% off, then another 20% off, is not 40% off. On $100: after the first discount you pay $80; 20% off that is $16, so you pay $64 — a total discount of 36%, not 40%.
Up and down by "the same" percent doesn't cancel
Increase $100 by 25% → $125. Now decrease by 20% → $100 again. But decrease by 25% → $93.75. The percentages that cancel each other aren't equal, because each one applies to a different amount.
These are reading traps as much as math traps — the same idea as the tax-and-discount example in Arithmetic Reasoning Practice Test.
Practice without a calculator
These conversions are useful in both math sections of the AFQT: in word problems (Arithmetic Reasoning) and in direct questions (Mathematics Knowledge). Our free practice test covers both, plus the two verbal sections, with the computer version's time limits — and ends with an AFQT estimate, a practice estimate rather than an official score.
For translating percent sentences in word problems ("what percent of A is B?"), see ASVAB Word Problems: How to Translate English Into Math.
Quick answers
What is 3/8 as a percent?
37.5%. One eighth is 12.5%, and three times that is 37.5%.
How do I find 15% of a number without a calculator?
Take 10% (move the decimal one place left), add half of it (that's 5%). For 80: 8 + 4 = 12.
Is a 20% discount followed by another 20% the same as 40% off?
No. On $100 you'd pay $64 — a total discount of 36%.
Can you use a calculator on the ASVAB?
No. The official ASVAB program doesn't allow calculators on any version of the test.
Sources
Link copied
Math
MathNew
MathNew