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      Question

      A sum of money amounts to Rs 2420 in 2 years and Rs 2662

      in 3 years at compound interest. Find the rate of interest per annum and the principal.
      A 10% and Rs.2000 Correct Answer Incorrect Answer
      B 18% and Rs.1000 Correct Answer Incorrect Answer
      C 13% and Rs.2100 Correct Answer Incorrect Answer
      D 21% and Rs.2050 Correct Answer Incorrect Answer

      Solution

      Let principal = P, rate = r% per annum. Amount after 2 years: Aβ‚‚ = P(1 + r)Β² = 2420 Amount after 3 years: A₃ = P(1 + r)Β³ = 2662 Divide A₃ by Aβ‚‚: A₃/Aβ‚‚ = (1 + r)Β³ / (1 + r)Β² = 1 + r So 2662/2420 = 1 + r Check ratio: 2420 Γ— 1.1 = 2662 β‡’ 2662/2420 = 1.1 Thus 1 + r = 1.1 β‡’ r = 0.1 = 10% Now P(1.1)Β² = 2420 P(1.21) = 2420 β‡’ P = 2420 / 1.21 = 2000 Rate = 10% p.a., Principal = Rs 2000.

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