A maths plan for SSC CHSL

The maths in SSC CHSL is not hard in the way a board exam's toughest questions are hard. It is school maths under a stopwatch. In Tier 1 you get about 15 minutes for 25 questions, around 36 seconds each. The candidates who score well are rarely maths toppers. They are the ones who practised the standard question types until the method became automatic, and who understood why their shortcuts work, so they never apply one in the wrong place.

How much maths is worth

In Tier 2, maths is a quarter of your 360 merit marks. In both tiers, arithmetic (percentage, ratio, average, profit and loss, interest, time and work, speed and distance) forms the backbone, with algebra, geometry, mensuration, trigonometry and data interpretation around it. Build your own picture of the mix from previous papers rather than trusting any single chart.

The topic order

If you have less than twelve weeks, compress weeks 1–7 first and keep them. Arithmetic is where the dependable marks are.

The learn–practise–time cycle

For every topic:

1. Learn: understand the method through three or four solved examples.
2. Practise: 40–60 questions untimed, checking each one.
3. Time: 15 questions in 10 minutes. If accuracy falls below about 80%, go back to step 2.

Every day, also do 15 minutes of calculation drills (tables to 20, squares to 30, cubes to 15, fraction-to-percentage conversions) and ten mixed questions from earlier topics, so nothing goes cold.

Five shortcuts, and why they work

1. Fractions for percentages. Learn this table until it is instant:

Why it works: a percentage is just a fraction with denominator 100, so 12.5% of 640 is 640 ÷ 8 = 80, with no multiplication at all.

2. Successive percentage change: a + b + ab/100. Two changes of a% and b% (use negatives for decreases) give a net change of a + b + ab/100.
Why: multiplying by (1 + a/100)(1 + b/100) gives 1 + a/100 + b/100 + ab/10,000. As a percentage, that is a + b + ab/100.

3. Take 100 or take the LCM. When a question gives only percentages or ratios, assume a convenient total: 100 for percentage questions, and the LCM of the given days for work questions.
Why: the answer is a ratio or a percentage, so it doesn't depend on the actual size you pick.

4. km/h to m/s: multiply by 5/18.
Why: 1 km/h = 1,000 m ÷ 3,600 s = 5/18 m/s.

5. Use the options. If an equation is hard to solve, substitute the options. If the options are far apart, estimate.
Why: a multiple-choice question only needs you to identify the right option, not to derive it.

Six worked questions

Q1. A price rises by 20% and then falls by 20%. What is the net change?
Using a + b + ab/100: 20 + (−20) + (20 × −20)/100 = 0 − 4 = −4%, a 4% decrease.
Check: 100 → 120 → 120 × 0.8 = 96.

Q2. A : B = 2 : 3 and B : C = 4 : 5. Find A : B : C.
Make B the same in both: multiply the first by 4 and the second by 3. A : B = 8 : 12 and B : C = 12 : 15. So A : B : C = 8 : 12 : 15.

Q3. The average of five numbers is 24. When one number is removed, the average of the rest is 22. Find the removed number.
Total of five = 5 × 24 = 120. Total of four = 4 × 22 = 88. Removed number = 120 − 88 = 32.

Q4. A can finish a job in 12 days and B in 18 days. How long will they take together?
Take the work as LCM(12, 18) = 36 units. A does 36 ÷ 12 = 3 units a day and B does 36 ÷ 18 = 2 units a day. Together they do 5 units a day, so they need 36 ÷ 5 = 7.2 days (7⅕ days).

Q5. A train 300 m long runs at 72 km/h. How long does it take to pass a pole?
72 × 5/18 = 20 m/s. To pass a pole, the train covers its own length: 300 ÷ 20 = 15 seconds.

Q6. Find the simple interest on ₹5,000 at 8% a year for 3 years.
SI = P × R × T ÷ 100 = 5,000 × 8 × 3 ÷ 100 = ₹1,200.

Common errors

Practice

1. What is 12.5% of 640?
2. A price rises from ₹250 to ₹300. What is the percentage increase?
3. Divide ₹1,800 in the ratio 4 : 5.
4. Find the average of 12, 15, 18 and 23.
5. The price of sugar rises by 25%. By what percentage must a family cut consumption to keep spending the same?
6. An article costs ₹640. At what price must it be sold for a 25% profit?
7. A car covers 180 km in 3 hours 36 minutes. What is its speed?
8. Find the compound interest on ₹10,000 at 10% a year for 2 years, compounded annually.

Answers:

1. 80. 12.5% = 1/8, and 640 ÷ 8 = 80.
2. 20%. Increase 50 on a base of 250: 50/250 = 1/5 = 20%.
3. ₹800 and ₹1,000. One part = 1,800 ÷ 9 = 200, so 4 × 200 and 5 × 200.
4. 17. Total = 68; 68 ÷ 4 = 17.
5. 20%. Cut = 25/(100 + 25) × 100 = 20%. Check: 1.25 × 0.8 = 1.
6. ₹800. 640 × 1.25 = 800 (or 640 + 640/4).
7. 50 km/h. 3 h 36 min = 3.6 h, and 180 ÷ 3.6 = 50.
8. ₹2,100. Amount = 10,000 × 1.1 × 1.1 = 12,100, so CI = 2,100. Equivalently 10 + 10 + 100/100 = 21% of 10,000.

A self-check after twelve weeks

• I can solve basic percentage and ratio questions in under 40 seconds.
• I know squares to 30, cubes to 15 and the fraction–percentage table.
• I have practised every topic in the table at least once, timed.
• My accuracy on mixed sets of 25 is above 80%.
• I keep an error log and redo its questions weekly.

Start with the first three topic guides: number system, HCF and LCM and simplification.