There is no algebra beyond the basics in SSC GD Part C, no trigonometry and no calculus. The difficulty is entirely speed: 20 questions inside the roughly 15 minutes you can spare for maths.
These are the specific things that turn a 90-second calculation into a 10-second one.
The fraction table
This is the single highest-value thing to memorise in the whole paper. Convert between fractions and percentages instantly:
| Fraction | % | Fraction | % |
|---|---|---|---|
| 1/2 | 50% | 1/8 | 12.5% |
| 1/3 | 33⅓% | 1/9 | 11⅑% |
| 1/4 | 25% | 1/10 | 10% |
| 1/5 | 20% | 1/12 | 8⅓% |
| 1/6 | 16⅔% | 1/16 | 6.25% |
| 1/7 | 14²/₇% | 1/20 | 5% |
“Find 12.5% of 640” becomes “one-eighth of 640” — 80, in two seconds, with no working. “35% of 200” becomes 3.5 × 20 = 70.
Assume the cost price is 100
For any profit, loss or discount question where no actual value is given, set CP = 100. Percentages become plain arithmetic.
An item is marked 40% above cost and sold at 25% discount. Profit?
CP = 100 → MP = 140 → SP = 140 × 0.75 = 105 → profit 5 → 5%.
No algebra, no fractions of unknowns.
The two speed conversions
- km/h → m/s: × 5/18
- m/s → km/h: × 18/5
And the distinction that is the whole question in train problems:
- Crossing a pole: distance = train length
- Crossing a platform or bridge: distance = train length + platform length
- Two trains opposite directions: relative speed adds
- Two trains same direction: relative speed subtracts
Percentage change is not symmetric
A 20% rise followed by a 20% fall does not return you to the start. 100 → 120 → 96.
The formula for successive changes of a% then b%:
net = a + b + (ab ÷ 100)
Check: 20 − 20 − 400/100 = −4%. Enter decreases as negatives.
Related trap: if a price rises by 25%, consumption must fall by 20%, not 25%, to keep spending flat. The formula is increase ÷ (100 + increase) × 100.
Percentage is always on the original
- Profit and loss percentages are on the cost price
- Discount is on the marked price
- Percentage change is on the original value
Most wrong answers in this topic come from dividing by the wrong base, and the exam always offers the wrong-base answer as an option.
Averages become subtraction
Convert averages to sums immediately: sum = average × count.
Average of 5 numbers is 65. Remove one and it becomes 60. Which was removed?
325 − 240 = 85. Two multiplications and a subtraction.
Also: the average of consecutive or equally spaced numbers is simply the middle one. The average of the first n natural numbers is (n+1)/2.
Work and time
For two workers: (a × b) ÷ (a + b).
A does it in 10 days, B in 15 → (10 × 15) ÷ 25 = 6 days.
For “how many workers” questions, use total work in worker-days: 8 men × 12 days = 96, so 6 days needs 96 ÷ 6 = 16 men.
Scaling
- Double a length → area × 4, because area goes with the square
- Double a length → volume × 8, because volume goes with the cube
The exam offers both 4 and 8 as options every time, so knowing which applies is the entire question.
Use π = 22/7 whenever the radius is a multiple of 7 — the sevens cancel and the arithmetic becomes clean.
Divisibility, for elimination
| Divisible by | Test |
|---|---|
| 3 | digit sum divisible by 3 |
| 4 | last two digits divisible by 4 |
| 8 | last three digits divisible by 8 |
| 9 | digit sum divisible by 9 |
| 11 | alternating digit sums differ by 0 or a multiple of 11 |
BODMAS, correctly
Division and multiplication have equal priority — work left to right, not division first. Same for addition and subtraction.
15 ÷ 3 × 2 + 4 = 5 × 2 + 4 = 14, not 6.5. This single misunderstanding accounts for a large share of simplification errors.
How to actually get faster
Knowing these is not the same as using them under pressure. Do timed sets of 15 questions, aiming for under 45 seconds each, and after every set look at which questions took longest. The answer is almost always a shortcut you knew but did not reach for.
Speed in Part C comes from recognition, not from calculating faster.