Every aptitude exam is designed to trick us. Not in an unfair way — but by presenting problems that LOOK like one thing and ARE something else. The questions below represent the most common traps that catch even prepared candidates. Learn these traps once, and we’ll never fall for them again.
Trap 1: “Increase BY” vs “Increase TO”
This trips up more people than any other trap.
These mean the same thing, but in a fast exam our brain can confuse “increased BY 20%” with “is now 20%.” Always pause and think: is 20% the CHANGE or the NEW VALUE?
Trap Example:
If a population increases by 20% in the first year and decreases by 20% in the second year, what is the net change?
Wrong answer (the trap): +20% then -20% = 0% change. Population stays the same.
Right answer: Let population = 100. After +20%: 120. After -20% of 120: 120 × 0.8 = 96. Net change = -4%. The population DECREASED by 4%.
In simple language, the 20% decrease applies to the BIGGER number (120), not the original (100). So it removes more than what was added.
Trap 2: Percentage OF vs Percentage CHANGE
- “What percentage IS A of B?” → (A/B) × 100
- “What is the percentage CHANGE from A to B?” → ((B-A)/A) × 100
Trap Example:
A is 25% more than B. B is what percentage less than A?
Wrong answer: B is 25% less than A.
Right answer: If B = 100, then A = 125. B is less than A by 25. Percentage less = (25/125) × 100 = 20%
The base changes! When we say “A is 25% more than B,” the base is B. When we ask “B is how much less than A,” the base is A. Different bases give different percentages.
Trap 3: Average Speed ≠ Average of Speeds
We covered this in the Averages topic, but it’s SO common as a trap that it deserves another mention.
Trap Example:
A car goes from A to B at 40 km/h and returns at 60 km/h. What is the average speed?
Wrong answer: (40 + 60) / 2 = 50 km/h
Right answer: Average speed = 2 × 40 × 60 / (40 + 60) = 4800/100 = 48 km/h
For equal DISTANCES, average speed is the harmonic mean: 2ab/(a+b). The simple average (a+b)/2 only works for equal TIMES.
Trap 4: Ratio Comparison
Trap Example:
Which is larger: 3/4 or 5/7?
The trap: We might glance and think 5/7 is bigger because 5 > 3 and 7 > 4. But that logic is wrong.
Right approach: Cross multiply: 3 × 7 = 21 and 4 × 5 = 20. Since 21 > 20, 3/4 > 5/7.
Or convert: 3/4 = 0.75, 5/7 = 0.714. So 3/4 is larger.
General rule for comparing a/b and c/d: If ad > bc, then a/b > c/d.
Trap 5: BODMAS Violations
Trap Example:
Simplify: 8 + 4 × 3 - 2 ÷ 2
Wrong answer (left to right): 8 + 4 = 12, × 3 = 36, - 2 = 34, ÷ 2 = 17
Right answer (BODMAS):
- Division: 2 ÷ 2 = 1
- Multiplication: 4 × 3 = 12
- Addition: 8 + 12 = 20
- Subtraction: 20 - 1 = 19
The order is: Brackets, Orders (powers), Division/Multiplication (left to right), Addition/Subtraction (left to right).
Trap 6: Reading the Question Wrong
This is the #1 source of errors, and it’s not even about math.
Trap Example:
A shopkeeper sells an item at 20% profit. If the cost price is Rs 500, find the COST PRICE.
Wait — the question already GIVES us the cost price (500) and asks for… the cost price. The answer is 500. But if we’re rushing, we might calculate the selling price (600) and pick that as the answer. Many exam-setters deliberately plant the selling price as an option.
Another example:
“How many MORE boys than girls…” — We need the DIFFERENCE, not the count of boys.
“What is the ratio of A to B?” — They want A:B, not B:A. Getting the ratio backwards is an extremely common error.
Trap 7: Unit Mismatch
Trap Example:
A train travels at 72 km/h. How many meters does it cover in 15 seconds?
The trap: Mixing up km/h and m/s.
Conversion: 72 km/h = 72 × (5/18) = 20 m/s
Distance in 15 seconds = 20 × 15 = 300 meters
If we forget to convert: 72 × 15 = 1080, which is nonsensical. Always check units.
Another classic: converting minutes to hours. 45 minutes = 0.75 hours, NOT 0.45 hours. Our decimal brain wants to write 0.45, but 45/60 = 0.75.
Trap 8: The “Too Easy” Trap
If a question on a hard section gives us an answer in 10 seconds, something might be wrong. Exam setters sometimes make the obvious (wrong) answer very easy to reach.
Trap Example:
A can do a job in 10 days. B can do it in 15 days. In how many days can they do it working together?
Trap answer (too easy): (10 + 15) / 2 = 12.5 days. This is the average, not the combined time.
Right answer: Combined rate = 1/10 + 1/15 = 3/30 + 2/30 = 5/30 = 1/6. Time = 6 days.
When two people work together, the combined time is ALWAYS less than the faster person’s time. If our answer is between the two individual times, it’s wrong.
Trap 9: Sign Errors with Negative Numbers
Trap Example:
Simplify: (-3)² vs -3²
- (-3)² = (-3) × (-3) = +9 (the square of negative 3)
- -3² = -(3²) = -(9) = -9 (the negative of 3 squared)
The parentheses make ALL the difference. Without them, the exponent applies only to 3, and the negative sign stays.
Another sign trap:
If the temperature drops from 5°C to -3°C, what is the change?
Change = -3 - 5 = -8°C (a drop of 8 degrees).
The trap is computing 5 - 3 = 2. We subtracted the magnitudes instead of using signed arithmetic.
Trap 10: Not Checking if the Answer is Reasonable
After solving, do a 2-second sanity check:
- Is the percentage between 0 and 100? (Unless the question explicitly allows more)
- Is the speed positive? Negative speed doesn’t make sense.
- Is the time positive? Can’t have negative time.
- Is the discount less than the price? A Rs 200 discount on a Rs 150 item is suspicious.
- Is the combined work time less than individual times? Always.
- Does the ratio make directional sense? If A earns more than B, the ratio A:B should be greater than 1.
Trap Example:
After a 30% discount, the price of a shirt is Rs 910. Find the original price.
If we compute 910 - 30% of 910 = 910 - 273 = 637, that’s the price AFTER a further 30% discount, not the original.
The correct approach: 70% of original = 910 → original = 910/0.7 = 1300
Sanity check: 30% of 1300 = 390. 1300 - 390 = 910 ✓
Trap Summary Cheat Sheet
Practice: Spot the Trap
Problem 1: A price rises by 25%, then falls by 20%. Is the final price the same, higher, or lower than the original?
Problem 2: Simplify: 12 ÷ 3 × 4 + 2 - 1
Problem 3: A is 50% more than B. What percentage of A is B?
Answers
Problem 1: Let original = 100. After +25%: 125. After -20% of 125: 125 × 0.8 = 100. The final price is the same as the original. (This is the rare case where it works out to the same! The trap here is people assuming the answer must be “different” because of the previous traps we discussed. In this specific case, 25% up and 20% down DO cancel. The formula: net factor = 1.25 × 0.80 = 1.00.)
Problem 2: BODMAS: Division and multiplication first, left to right. 12 ÷ 3 = 4. 4 × 4 = 16. Then 16 + 2 - 1 = 17. (Trap: doing left to right without BODMAS gives a different answer.)
Problem 3: If B = 100, A = 150. B as percentage of A = (100/150) × 100 = 66.67%. (Trap: saying 50% because we confuse “A is 50% more than B” with “B is 50% less than A.” B is actually 33.33% less than A, and B is 66.67% OF A.)