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Q44·CSAT · Prelims 2023

Comparing compound interest rates (Half-yearly vs Annually)

DI / DSComparative QuantitativeComparative quantitativeEasy

Question

A principal P becomes Q in 1 year when compounded half-yearly with R% annual rate of interest. If the same principal P becomes Q in 1 year when compounded annually with S% annual rate of interest, then which one of the following is correct?

Options

a

R = S

b

R > S

c

R < S

Answer
d

R ≤ S

Explanation

Write out the compound interest equations for a 1-year timeline under both scenarios[cite: 3744, 3745]:

1Compounded half-yearly (2 cycles, nominal rate divided by 2):

Q = P \left(1 + R/200\right)²

2Compounded annually (1 cycle) :

Q = P \left(1 + S/100\right)^1

Since the final amount Q and initial principal P are identical in both cases, equate the multipliers [cite: 3744, 3745]: \left(1 + R/200\right)² = 1 + S/100 1 + R/100 + R²/40000 = 1 + S/100 R/100 + R²/40000 = S/100

Because the term ⟨MATH⟩R²/40000⟨/MATH⟩ is strictly positive, it follows that ⟨MATH⟩S/100⟨/MATH⟩ must be strictly greater than ⟨MATH⟩R/100⟨/MATH⟩. This forces S > R, which can be rewritten as R < S.

Frequent compounding cycles add compounding interest. To match the same final amount, an annually compounded interest rate must have a higher face value (S > R).

Answer: (c).

Question details

Year

2023

Paper

CSAT

Question

Q44

Section

Data Interpretation & Sufficiency

Sub-topic

Comparative Quantitative

Type

Comparative quantitative

Difficulty

Easy

Source hint

Quantitative comparison

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