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A sum of money becomes Rs. 7200 in three years and it becomes Rs. 8640 after six years on compound interest, what is the sum invested?


1. Rs. 5400
2. Rs. 6000
3. Rs. 6600
4. Rs. 7000

1 Answer

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Best answer
Correct Answer - Option 2 : Rs. 6000

Given:

Amount after 3 years (A3) = Rs. 7200

Amount after 6 years (A6) = Rs. 8640

Formula used:

A = P × (1 + R/100)n

A is Amount, R is Rate percent per annum, n is time

Solution:

A6 = P × (1 + R/100)6 = 8640      ---- (1)

A3 = P × (1 + R/100)3 = 7200      ---- (2)

Dividing (1) by (2), we’ll get

\(⇒ \; \frac{{P \times {{\left( {1 + \frac{R}{{100}}} \right)}^6}}}{{P \times {{\left( {1 + \frac{R}{{100}}} \right)}^3}}} = \frac{{8640}}{{7200}}\)

⇒ (1 + R/100)3 = 6/5

Now putting the above value in (2) we’ll get

⇒ P × (6/5) = 7200

⇒ P = (7200 × 5)/6

⇒ P = 6000

Sum invested is Rs. 6000

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