10 ASVAB Math Traps — and How to Spot Each One
The mistakes that cost the most points in ASVAB math aren't hard math. Ten classic traps in word problems and algebra, each with a quick way to catch it.
Most lost points in ASVAB math don't come from math that's too hard. They come from a handful of predictable mistakes — misreading a sentence, applying the right rule to the wrong number, stopping one step early. On a multiple-choice test, the result of each mistake can sit right there among the answer choices, looking correct. Here are ten classic traps, with a one-line check for each. (All examples are our own.)
1. "Less than" in the wrong order
"5 less than x" is x − 5, not 5 − x.
Check: replace x with 20. "5 less than 20" is 15 = 20 − 5.
2. Averaging two speeds
Driving 120 miles at 40 mph and back at 60 mph is not an average of 50 — it's 48, because you spend longer at the slower speed. Average speed is total distance ÷ total time.
Check: same distance at two speeds → the answer is below the midpoint. Full explanation: The Average Speed Trap.
3. Adding discounts
20% off and then another 20% off is not 40% off. On $100: $80, then $64 — a 36% discount.
Check: apply each percent to the amount it actually applies to, one step at a time.
4. The wrong base for percent change
From 40 to 50 is a 25% increase; from 50 to 40 is a 20% decrease.
Check: divide the change by the starting value.
5. Discount, then tax — combined
A $80 item at 25% off with 8% tax is $64.80, not 83% of $80 ($66.40). The discount applies to $80; the tax applies to $60.
Check: follow the order of the story. Worked out in Arithmetic Reasoning Practice Test.
6. Answering the wrong unknown
If Jake has 4 more than twice Maria's cards and together they have 31, solving gives Maria = 9 — but the question asked about Jake (22).
Check: before choosing, reread the last sentence of the question.
7. Distributing to only one term
4(x − 2) = 2x + 6 means 4x − 8 on the left, not 4x − 2. The right answer is x = 7; the slip gives x = 4.
Check: plug your answer back in. With x = 4, the left side is 8 and the right side is 14 — they don't match.
8. The negative-sign exponent
−3² = −9, but (−3)² = 9. Without parentheses, the exponent applies only to the 3.
Check: look for parentheses before you square. More in Exponents and Square Roots.
9. Adding squares when you know the hypotenuse
A 13-foot ladder with its base 5 feet from the wall reaches 12 feet up (√(169 − 25)), not √(169 + 25) ≈ 13.9 — longer than the ladder.
Check: a leg can never be longer than the hypotenuse. If you know the longest side, subtract. More in The Pythagorean Theorem on the ASVAB.
10. Mixing units — or ignoring what the remainder means
Two quieter traps:
- Units: 45 minutes is 0.75 hours, not 0.45. Convert everything to the same unit before calculating.
- Remainders in real life: 50 people, and each van holds 8. 50 ÷ 8 = 6.25 — so you need 7 vans, not 6. You can't leave a quarter of a van's passengers behind.
Check: ask whether the answer makes sense in the story.
The habits that catch all ten
- Read the question sentence first, and again before you answer.
- Write the setup before you calculate.
- Estimate — and compare your final answer with the estimate.
- Plug your answer back in whenever the problem allows it.
These habits fit the time you have: on the computer version, Arithmetic Reasoning gives you 55 minutes for 15 scored questions, and Mathematics Knowledge 31 minutes for 15 — without a calculator.
Practice it
Our free practice test includes both math sections with the computer version's time limits, plus Word Knowledge and Paragraph Comprehension, and ends with an AFQT estimate — a practice estimate, not an official score. When you miss a question, look for which of these ten traps it was.
Quick answers
What kind of mistakes cost points in ASVAB math?
Many are reading mistakes rather than math mistakes: subtracting in the wrong order, averaging two speeds, adding percents, or answering for the wrong unknown.
How do I avoid careless mistakes on ASVAB math?
Write the setup before calculating, estimate the answer, and plug your result back into the problem when you can.
Is 25% off and then 8% tax the same as 17% off?
No. The discount and the tax apply to different amounts. On $80, you'd pay $64.80, not $66.40.
Can I use a calculator to check my answers?
No. Calculators aren't allowed on any version of the ASVAB.
Sources
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