In this guide
The SSC GD computer-based exam has 80 questions in 60 minutes, with no separate time limit for each section (as reported for recent cycles; check the latest notification). That is about 45 seconds a question on average. Reasoning and GK questions often take less, which leaves a little extra for maths, but not much.
In that time, the method is rarely the problem. The arithmetic is. A candidate who multiplies 36 × 25 in their head in three seconds has time to check the answer. One who does long multiplication on the rough sheet does not. The tricks below are simple, reliable, and quick to learn.
First, memorise the basics
- Tables up to 20.
- Squares up to 30 and cubes up to 15. See squares and square roots.
- Fraction–percentage values (table below).
Ten minutes a day for two weeks is enough. Everything else in this post builds on these.
| Fraction | % | Fraction | % |
|---|---|---|---|
| 1/2 | 50% | 1/8 | 12.5% |
| 1/3 | 33⅓% | 3/8 | 37.5% |
| 1/4 | 25% | 1/9 | 11 1/9% |
| 1/5 | 20% | 1/10 | 10% |
| 1/6 | 16⅔% | 1/12 | 8⅓% |
| 1/7 | 14 2/7% | 1/20 | 5% |
Multiplication tricks
| Multiply by | Do this | Example |
|---|---|---|
| 5 | × 10, then halve | 64 × 5 = 640 ÷ 2 = 320 |
| 25 | × 100, then ÷ 4 | 36 × 25 = 3,600 ÷ 4 = 900 |
| 9 | × 10, then subtract the number | 47 × 9 = 470 − 47 = 423 |
| 99 | × 100, then subtract the number | 68 × 99 = 6,800 − 68 = 6,732 |
| 11 (two-digit) | Put the digit sum in the middle | 23 × 11 = 2 (5) 3 = 253 |
For × 11, carry if the middle sum is 10 or more: 57 × 11 → 5 (12) 7 → 627.
Squaring numbers ending in 5
Multiply the first digit by the next number and write 25 after it: 55² → 5 × 6 = 30 → 3,025.
Two numbers near 100
Find how far each is below 100. Subtract one gap from the other number for the first two digits. Multiply the gaps for the last two digits.
- 96 × 98: gaps 4 and 2. 96 − 2 = 94. 4 × 2 = 08. Answer 9,408.
Same distance either side
(a + b)(a − b) = a² − b². So 104 × 96 = 100² − 4² = 10,000 − 16 = 9,984.
Division tricks
- ÷ 5: multiply by 2 and divide by 10. 135 ÷ 5 = 270 ÷ 10 = 27.
- ÷ 25: multiply by 4 and divide by 100. 350 ÷ 25 = 1,400 ÷ 100 = 14.
- ÷ 8 is half of half of half: 480 ÷ 8 → 240 → 120 → 60.
Quick percentages
- 10%: move the decimal one place left. 10% of 370 = 37.
- 5%: half of 10%. 18.5.
- 1%: move the decimal two places. 3.7.
- 15% = 10% + 5% = 37 + 18.5 = 55.5.
- Swap trick: x% of y = y% of x. So 16% of 25 = 25% of 16 = 4.
Checking your answer
Estimate. 49 × 21 is about 50 × 20 = 1,000. If you get 10,290, something is wrong (the exact answer is 1,029).
Last digit. The last digit of a product depends only on the last digits: 37 × 24 must end in 8, because 7 × 4 = 28. Any option ending in another digit is wrong.
Digit sum (casting out nines). Add the digits of each number until one digit remains. The digit sum of a product equals the digit sum of the product of the digit sums.
Solved examples
Example 1. What is 12.5% of 480?
- 12.5% = 1/8.
- 480 ÷ 8 = 60.
Example 2. 25 notebooks cost ₹36 each. Find the total.
- 36 × 25 = 3,600 ÷ 4.
- = ₹900.
Example 3. Find 98 × 97.
- Gaps from 100: 2 and 3.
- 98 − 3 = 95. 2 × 3 = 06.
- Answer: 9,506.
Example 4. Which option is 234 × 56: (a) 13,014 (b) 13,104 (c) 13,114 (d) 12,904?
- All end in 4, so the last-digit check does not help.
- Digit sums: 234 → 9; 56 → 11 → 2. 9 × 2 = 18 → 9.
- Option digit sums: (a) 9, (b) 9, (c) 10 → 1, (d) 16 → 7. So (c) and (d) are out.
- To choose between (a) and (b), split: 234 × 56 = 234 × 50 + 234 × 6 = 11,700 + 1,404 = 13,104.
- Answer: (b) 13,104.
Example 5. Find 15% of 640 in your head.
- 10% = 64. 5% = 32.
- 15% = 96.
Common mistakes
| Mistake | Correct way |
|---|---|
| Using the × 11 trick without carrying | If the middle sum is 10 or more, carry 1 |
| Writing 98 × 96 as 948 instead of 9,408 | The gap product always takes two places (08) |
| Trusting a trick without a check | Estimate every answer once |
| Learning twenty tricks and using none | Master five and use them daily |
A daily 10-minute drill
| Minutes | Drill |
|---|---|
| 0–3 | 10 multiplications using the tricks above |
| 3–6 | 10 squares or cubes from memory |
| 6–10 | 10 quick percentages and fraction values |
Practice set
- 48 × 5
- 44 × 25
- 36 × 11
- 85²
- 15% of 240
- 37.5% of 640
- 97 × 95
- 68 × 99
Answers:
- 480 ÷ 2 = 240.
- 4,400 ÷ 4 = 1,100.
- 3 (9) 6 = 396.
- 8 × 9 = 72, then 25: 7,225.
- 24 + 12 = 36.
- 37.5% = 3/8. 640 ÷ 8 × 3 = 240.
- Gaps 3 and 5. 97 − 5 = 92; 3 × 5 = 15. 9,215.
- 6,800 − 68 = 6,732.
What to do next
- Learn the fraction–percentage table this week.
- Run the 10-minute drill every day for three weeks.
- In each mock, check two answers by last digit or estimation.
- Apply the tricks inside percentage questions, where they save the most time.
A note on dates and numbers. Exam patterns, vacancies and schedules change from year to year. Always confirm the current details in the latest notification on the Staff Selection Commission website .
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