Quantitative Aptitude practice

Simple Interest Questions with Answers

Simple interest is interest calculated only on the original sum for the whole period. Simple interest questions in SSC CGL, CHSL and MTS, RRB NTPC and Group D, and bank prelims ask for the interest, the rate, the time or the principal, and sometimes a sum split into two parts. The formula is short, so speed and accuracy matter here. Tip: convert the time into years (months ÷ 12) and the rate into a per-year figure before you substitute.

Formulas you need

Simple Interest (SI) = P × R × T ÷ 100
Amount (A) = P + SI = P × (1 + RT ÷ 100)
P = SI × 100 ÷ (R × T)
R = SI × 100 ÷ (P × T)
T = SI × 100 ÷ (P × R)
A sum becomes n times itself in T years: R = (n − 1) × 100 ÷ T
For a given sum, SI is directly proportional to rate × time
If two parts of a sum earn equal SI, then P₁ × R₁ × T₁ = P₂ × R₂ × T₂
Time in months must be divided by 12 before using the formula

Worked examples

Q1. Find the simple interest on ₹8,000 at 7.5% per annum for 4 years.

SI = P × R × T ÷ 100
= 8,000 × 7.5 × 4 ÷ 100
= 80 × 30 = 2,400

Answer: ₹2,400

Q2. A sum of ₹6,000 amounts to ₹7,260 in 3 years at simple interest. Find the rate per annum.

SI = 7,260 − 6,000 = ₹1,260
R = SI × 100 ÷ (P × T)
= 1,260 × 100 ÷ (6,000 × 3)
= 126,000 ÷ 18,000 = 7

Answer: 7% per annum

Q3. ₹9,000 is divided into two parts. The first part is lent at 5% for 4 years and the second at 8% for 5 years, and both earn the same simple interest. Find the two parts.

Equal SI means P₁ × 5 × 4 = P₂ × 8 × 5
20 P₁ = 40 P₂, so P₁ : P₂ = 2 : 1
P₁ = 9,000 × 2 ÷ 3 = ₹6,000; P₂ = ₹3,000
Check: 6,000 × 5% × 4 = 1,200 and 3,000 × 8% × 5 = 1,200

Answer: ₹6,000 and ₹3,000

Shortcuts that save time

For 'sum doubles or triples' questions, use R × T = (n − 1) × 100. A sum that doubles in 8 years gives R = 100 ÷ 8 = 12.5%, with no principal needed.
When the question gives amount instead of interest, subtract the principal first. Mixing up amount and interest is the usual way marks are lost here.
For equal-interest splits, compare R × T for the two parts and divide the sum in the inverse ratio. This avoids setting up an equation under time pressure.

Where this appears

SSC CGL Tier-1	Quantitative Aptitude section
SSC CHSL Tier-1	Quantitative Aptitude section
SSC MTS	Numerical and Mathematical Ability section
RRB NTPC CBT-1	Mathematics section
Bank PO/Clerk Prelims	Quantitative Aptitude section

Simple Interest

Practice Set

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Find the simple interest on Rs. 13000 at 12% per annum for 8 years.

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