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      Question

      An amount becomes Rs. 9,360 in 3 years when invested at

      simple interest at the rate of r% per annum. If the interest rate had been (r + 4)% per annum, the same amount would have increased to Rs. 10,200 in the same period. What is the initial sum invested?
      A Rs.7000 Correct Answer Incorrect Answer
      B Rs.5720 Correct Answer Incorrect Answer
      C Rs.6620 Correct Answer Incorrect Answer
      D Rs.4700 Correct Answer Incorrect Answer

      Solution

      ATQ,

      Let the principal be Rs. ‘P’.

      Simple interest at r% = 9360 – P

      So, 9360 – P = P × 3 × r ÷ 100

      Or, 100 × (9360 – P) = 3Pr ………. (I)

      Now, interest at (r + 4)% = 10200 – P

      So, 10200 – P = P × (r + 4) × 3 ÷ 100

      Or, 100 × (10200 – P) = 3Pr + 12P ………. (II)

      From equations (I) and (II),

      1020000 – 100P = 936000 – 88P

      Or, 84000 = 12P

      So, P = 7000

      Hence, the original sum invested = Rs. 7,000

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