The 8 GMAT word-problem traps everyone falls for
The arithmetic is rarely the problem. These eight set-up errors are — each with a worked example and the habit that prevents it.
If your word-problem accuracy has stopped improving, the cause is almost never that you do not know enough algebra. GMAT word problems reuse a small number of set-up errors, and the wrong answer choices are built to catch exactly those errors.
Each trap below comes with a short example showing the wrong route, the right one, and a habit that stops it happening again.
1. Drawing without replacement
When items are drawn one after another and not put back, the pool shrinks. The second draw has one fewer item in the denominator, and — if the first draw was the kind you want — one fewer in the numerator too.
A box holds 7 red and 13 blue marbles. Two are drawn without replacement. What is the probability both are red?
Trap: (7/20)² = 49/400 — treats the draws as independent.
Right: 7/20 × 6/19 = 42/380 = 21/190.
Before any arithmetic, look for "without replacement", "remaining", or "the second one". If you find one, reduce the pool for every later draw.
2. Mixed units
Questions switch units mid-sentence — minutes and hours, cents and dollars, per day and per week — and some answer choices are what you get by carrying the wrong unit through.
A pump moves 45 litres per minute. How many litres does it move in 2.5 hours?
Trap: 45 × 2.5 = 112.5 — hours used as if they were minutes.
Right: 2.5 hours is 150 minutes, and 45 × 150 = 6,750 litres.
Write every quantity with its unit before you set up an equation, and convert first.
3. The wrong base for a percent change
Percent change is (new − original) ÷ original. The denominator is the original value — not the new one, and not the average of the two.
A price falls from 80 to 60. By what percent did it fall?
Trap: 20 ÷ 60 ≈ 33% — divides by the new price.
Right: 20 ÷ 80 = 25%.
Say "percent of what?" out loud and name the base before you divide.
4. Adding successive percentages
A second percentage change applies to the value after the first, not to the starting value. Successive changes multiply.
A stock rises 25% and then falls 20%. What is the net change?
Trap: +25% − 20% = +5%.
Right: 1.25 × 0.80 = 1.00 — no net change at all.
Turn every percent change into a multiplier and chain them. If an answer choice equals the sum of two percentages in the question, treat it as a trap.
5. Averaging speeds
Average speed is total distance over total time. It equals the average of two speeds only when equal amounts of time are spent at each — and word problems almost always give equal distances instead.
A cyclist rides 30 km out at 15 km/h and back at 10 km/h. What is the average speed?
Trap: (15 + 10) ÷ 2 = 12.5 km/h.
Right: 60 km ÷ (2 h + 3 h) = 12 km/h.
Never average two rates. Find total distance and total time separately, then divide once.
6. Answering the wrong unknown
The quantity you solve for is often not the quantity asked for. A choice matching your intermediate value is there for anyone who stops one step early.
The sum of three consecutive integers is 72. What is the largest?
Trap: 23 — the value of n in n + (n+1) + (n+2) = 72.
Right: n = 23, so the largest is n + 2 = 25.
Underline what the question asks for before you start, and re-read it before you choose.
7. Counting points instead of intervals
Change spread across a period happens over the gaps between measurements, not over the measurements themselves. Five yearly figures span four years of change.
Headcount was 120 at the end of 2019 and 216 at the end of 2023. What was the average yearly increase?
Trap: 96 ÷ 5 = 19.2 — divides by the number of years listed.
Right: 96 ÷ 4 = 24 — there are four one-year intervals.
Before dividing a total change, count the gaps, not the endpoints.
8. Reading ratio numbers as amounts
A ratio of 3 : 5 describes a relationship, not a quantity. The real amounts are 3k and 5k for some multiplier k, and the question almost always gives you enough to find k.
Boys and girls are in the ratio 3 : 5, with 16 more girls than boys. How many students are there?
Trap: 3 + 5 = 8.
Right: 5k − 3k = 16, so k = 8, and the class has 8k = 64 students.
Write every part of a ratio as a multiple of k the moment you see it.
None of these traps is about difficult mathematics. Each is a moment where the question invites a quick move that looks right. The habit that beats all eight is the same: slow down for ten seconds at the set-up, because that is where these questions are won or lost.
Every Medharu practice question comes with a full explanation, and the wrong answers are written to catch exactly the mistakes above — so you learn which one you made.
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