It will Amount to Eight Times Itself at the Same Rate of Interest in :

A sum of money placed at compound interest doubles itself in 5 years. It will amount to eight times itself at the same rate of interest in

  1. 12 years
  2. 15 years
  3. 20 years
  4. 10 years
Anurag Mishra Professor Asked on 26th October 2015 in Maths.
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  • 1 Answer(s)

    Answer is (2): 15 years

    Explanation:-
    Let the principal be Rs.1 which become after Rs.2 after 5 years
    Then 2 = 1(1 + R/100)5 ………………………… (1)

    Let the principal of Rs.1 becomes Rs.8 after n years
    Then, 8 = 1(1 + R/100)n ………………………  (2)
    (2)3 = 1 (1 + R/100)n
    Comparing equation (1) & (2)
    Then, [1(1 + R/100)5]3 = 1 (1 + R/100)n
    1 (1 + R/100)15 = 1 (1 + R/100)n
    n = 15

    Then, the amount to eight times itself at the same rate of interest in 15 years.

    Hence, the answer is (2) 15 years.
     

    Monis Rasool Professor Answered on 28th October 2015.
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