Common maths mistakes in SSC GD

Look at any mock test you got wrong and sort the maths errors into two piles: "I did not know how" and "I knew how but slipped". For most candidates the second pile is bigger. Those are the cheapest marks to win back, because you do not need to learn anything new. You only need better habits.

The SSC GD paper also deducts 0.25 marks for each wrong answer (as reported; check the latest notification). A slip does not just lose you 2 marks. It costs you 2.25 compared with getting it right.

Here are the ten mistakes that turn up most often in SSC GD maths, each with an example and a habit that fixes it.

1. Percentage of the wrong number

A price rises from ₹200 to ₹250. The increase is ₹50. Dividing by 250 gives 20%. The correct answer is 50 ÷ 200 × 100 = 25%.

Habit: percentage change is always change ÷ original × 100. Write "÷ original" on the rough sheet before you divide.

2. BODMAS errors

20 + 10 × 2. Adding first gives 60. The correct answer is 20 + 20 = 40.

Another trap: 48 ÷ 4 × 2. Division and multiplication go left to right, so 12 × 2 = 24, not 48 ÷ 8 = 6.

Habit: brackets, then "of", then ÷ and × from left to right, then + and − from left to right. See simplification.

3. Unit mistakes

A 100 m train runs at 36 km/h. How long does it take to pass a pole? Dividing 100 by 36 mixes metres with km/h. Convert: 36 × 5/18 = 10 m/s, so the time is 10 seconds.

Habit: convert units before calculating. Minutes to hours means ÷ 60, and 1 m² is 10,000 cm², not 100.

4. Radius versus diameter

A circle has a diameter of 14 cm. Using 14 as r gives an area of 616 cm². The radius is 7, so the area is 22/7 × 7 × 7 = 154 cm².

Habit: underline the word "radius" or "diameter" in every circle or cylinder question. See mensuration.

5. Profit on the wrong price

An item costing ₹200 is sold for ₹250. The profit is ₹50. Profit % is 50 ÷ 200 × 100 = 25%, not 50 ÷ 250 × 100 = 20%.

Habit: profit and loss % are on cost price, unless the question clearly says otherwise.

6. Discount on the wrong price

An item is marked ₹800 and sold at a 15% discount. The discount is 15% of 800 = ₹120, so the selling price is ₹680.

Habit: discount % is always on the marked price.

7. Adding percentages that follow each other

A price rises 10% and then falls 10%. Many answer "no change". Work it through: 100 → 110 → 99. That is a 1% fall.

Habit: for two changes of a% and b%, the net change is a + b + (a × b ÷ 100), with a minus sign on any fall. Here: 10 − 10 − 1 = −1%.

8. Wrong average speed

Going at 30 km/h and returning at 60 km/h is not an average of 45 km/h. For the same distance each way, average speed = 2 × 30 × 60 ÷ 90 = 40 km/h.

Habit: average speed is always total distance ÷ total time.

9. Misreading the question

Words like not, except, more than, less than, remaining and per cent of the remaining change the answer. So does "how much more", which asks for a difference, not a total.

Habit: before solving, write on the rough sheet what the question asks for in two or three words, such as "B's share" or "new price".

10. Spending too long on one question

The paper gives about 45 seconds per question on average. Getting stuck on one hard question can cost you two easy ones.

Habit: if a maths question is not moving after about a minute, mark it for review, move on, and come back at the end.

Summary table

Keep a mistake log

After every mock, write each wrong maths answer in a notebook with three columns: the question, what you did, and the correct habit. After five mocks, count which mistake number from this list appears most. That is the one to fix first. A few lines after each test does more for your score than another full chapter.

Practice: find the mistake

1. "A price rises from ₹400 to ₹500, so the increase is 20%."
2. "30 − 10 ÷ 5 = 4."
3. "An item costing ₹200 is sold for ₹250, so the profit is 20%."
4. "A 5% rise followed by a 5% fall means no change."
5. "A 100 m train at 36 km/h passes a pole in 100 ÷ 36, about 2.8 seconds."
6. "A circle with a diameter of 14 cm has an area of 616 cm²."
7. "An item marked ₹800 at a 15% discount sells for ₹785."
8. "Going at 30 km/h and returning at 60 km/h gives an average speed of 45 km/h."

Answers:

1. Divide by the original: 100 ÷ 400 = 25%.
2. Division first: 30 − 2 = 28.
3. Profit on cost price: 50 ÷ 200 = 25%.
4. 100 → 105 → 99.75, a 0.25% fall.
5. 36 km/h = 10 m/s, so 10 seconds.
6. The radius is 7 cm, so the area is 154 cm².
7. 15% of 800 is 120, so the price is ₹680.
8. 2 × 30 × 60 ÷ 90 = 40 km/h.

What to do next

• Start a mistake log with the three columns above.
• Before your next mock, read the summary table once.
• In the mock, write "÷ original", "CP" or "MP" on the rough sheet as you set up each question.
• After the mock, find your most common mistake and practise ten questions on that topic.