Interest (SI & CI)
🔒 Log in to trackSimple and compound interest — a near-certain slot in every shift. Tier 1 favourites: direct SI, amount at CI with clean chips, CI–SI differences and doubling chains. Tier 2 adds instalments and multi-year rate cases. Everything runs on the multiplier-chip habit.
One page per subtopic: detailed notes, every question type, formulas, tricks and practice sets.
Every formula on one printable page, grouped by subtopic.
5 exam-level questions worked step by step.
68 questions — untimed practice or a timed test with analysis.
Track record in the exam
Questions per shift in recent SSC CGL papers.
Test difficulty mix (68 questions)
Question patterns exams keep repeating
Taken from previous-year papers. If a pattern is marked "very common", expect to see it in your exam.
Simple interest: find the missing value
very commonSimple interest, principal, rate or time is asked. "Doubles in 10 years" also belongs here.
Simple interest adds the same amount every year.
"Doubles" means the interest equals 100% of the sum.
A sum doubles in 10 years at simple interest. In how many years will it triple?
Doubling = 100% interest in 10 years, so 10% per year
Tripling = 200% interest
200 ÷ 10 = 20 years
Compound interest amount
very commonSum, rate and a few years are given, interest added to the sum each year.
Multiply by 1.1 for 10%, 1.2 for 20%, once for every year.
To find the sum, divide instead.
What sum becomes ₹6,655 in 3 years at 10% compound interest?
Each year × 1.1
3 years: 1.1 × 1.1 × 1.1 = 1.331
Sum × 1.331 = 6655
Sum = 6655 ÷ 1.331 = ₹5,000
Difference between CI and SI
very common"The difference between CI and SI for 2 years is ₹50."
For 2 years, the difference is the interest on one year's interest.
Work backwards to find the sum.
The difference between CI and SI for 2 years at 10% is ₹50. Find the sum.
Year 1 interest = 10% of sum
Difference = 10% of that = 1% of sum
1% of sum = 50, so sum = 50 × 100 = ₹5,000
Half-yearly or quarterly interest
very common"Compounded half-yearly" or "quarterly".
Halve the rate and double the number of periods.
For quarterly, quarter the rate and multiply periods by 4.
Find the compound interest on ₹10,000 at 20% per year for 1½ years, compounded half-yearly.
Rate per half-year = 10%; periods = 3
Amount = 10,000 × 1.1 × 1.1 × 1.1 = 13,310
CI = 13,310 − 10,000 = ₹3,310
Doubling and tripling chains
common"A sum doubles in 8 years. When will it become 8 times?"
At compound interest, each doubling takes the same time.
At simple interest, the interest grows by the same amount each year.
A sum doubles in 8 years at compound interest. In how many years will it become 8 times?
2 × 2 × 2 = 8, so it doubles 3 times
Each doubling takes 8 years
3 × 8 = 24 years
Equal yearly instalments
commonA loan is repaid in 2 or 3 equal yearly instalments.
Shrink each instalment back to today's value (÷ 1.1 for each year at 10%).
Add them up and set equal to the loan.
A loan of ₹3,310 at 10% compound interest is repaid in 3 equal yearly instalments. Find each instalment.
Value today = x ÷ 1.1 + x ÷ 1.21 + x ÷ 1.331
Multiply by 1.331: 1.21x + 1.1x + x = 3.31x, so value today = 3.31x ÷ 1.331
3.31x ÷ 1.331 = 3310, so x = 1000 × 1.331 = ₹1,331
Different rate each year
common"8% in the first year, 10% in the second."
Multiply year by year, using each year's own rate.
Never average the rates.
₹25,000 is invested at 10% in year 1 and 20% in year 2. Find the amount.
Year 1: 25,000 × 1.1 = 27,500
Year 2: 27,500 × 1.2 = ₹33,000
Find the rate from two amounts
commonAmounts after two years in a row are given.
The gain in the later year is one year's interest.
Compare it with the earlier amount.
A sum is ₹4,840 after 2 years and ₹5,324 after 3 years at compound interest. Find the rate.
Gain in year 3 = 5,324 − 4,840 = 484
484 is the interest on 4,840
484 ÷ 4,840 = 0.1 = 10%