NDA NA: Quantitative Aptitude — Simple & Compound Interest (2026)

Key Formulas

Simple Interest (SI): SI = P × R × T / 100  |  Amount = P + SI

Compound Interest (CI): A = P(1 + R/100)ⁿ  |  CI = A − P

Difference (CI−SI) for 2 years: = P(R/100)²

Difference (CI−SI) for 3 years: = P(R/100)²(3 + R/100)

⚡ Shortcut: For 10% for 2 years: CI is always 1% more than SI (as a % of principal × 2).

Common Patterns

  • Finding rate or time given SI/CI values
  • Comparing SI and CI
  • Half-yearly/quarterly compounding
  • Doubling/tripling time

Practice MCQs — Simple & Compound Interest

Q1. SI on ₹5000 at 8% for 3 years?

  • A) ₹1000
  • B) ₹1200
  • C) ₹1400
  • D) ₹1600
Answer: B) ₹1200 — SI=5000×8×3/100=₹1200.

Q2. CI on ₹10000 at 10% for 2 years?

  • A) ₹1000
  • B) ₹1500
  • C) ₹2100
  • D) ₹2500
Answer: C) ₹2100 — CI=10000[(1.1)²-1]=10000×0.21=₹2100.

Q3. Difference between CI and SI on ₹1000 at 10% for 2 years?

  • A) ₹10
  • B) ₹20
  • C) ₹30
  • D) ₹40
Answer: A) ₹10 — SI=200; CI=210; diff=₹10.

Q4. At what rate SI doubles in 8 years?

  • A) 10%
  • B) 12.5%
  • C) 14%
  • D) 16%
Answer: B) 12.5% — 100=P×r×8/100 → r=12.5%.

Q5. ₹2000 becomes ₹2500 in 5 years SI. Rate?

  • A) 3%
  • B) 4%
  • C) 5%
  • D) 6%
Answer: C) 5% — 500=2000×r×5/100 → r=5%.

Q6. CI on ₹8000 at 5% for 3 years?

  • A) ₹1200
  • B) ₹1261
  • C) ₹1300
  • D) ₹1350
Answer: B) ₹1261 — 8000×(1.05)³=8000×1.157625=9261 → CI=₹1261.

Q7. Sum triples in 20 years SI. Rate?

  • A) 5%
  • B) 8%
  • C) 9%
  • D) 10%
Answer: D) 10% — 200=P×r×20/100 → r=10%.

Q8. Difference between CI (half-yearly) and SI at 10% for 1 year on ₹4000?

  • A) ₹10
  • B) ₹20
  • C) ₹30
  • D) ₹40
Answer: A) ₹10 — SI=400; CI half-yearly: 4000×(1.05)²-4000=4000×0.1025=₹410. Diff=₹10.

Q9. ₹1200 at 20% CI annually for 2 years and ₹1500 at SI. Which is higher?

  • A) SI
  • B) CI
  • C) Equal
  • D) Cannot determine
Answer: B) CI — CI=1200×1.44=1728; SI=1500+1500×20×2/100=1500+600=2100. SI higher.

Q10. In what time will ₹3000 amount to ₹3600 at 5% SI?

  • A) 2 yr
  • B) 3 yr
  • C) 4 yr
  • D) 5 yr
Answer: C) 4 yr — 600=3000×5×t/100 → t=4 years.