Question : If at compound interest a certain sum becomes 2 times itself in 5 years, then in how many years will it become 8 times itself?
Option 1: 18 years
Option 2: 12 years
Option 3: 20 years
Option 4: 15 years
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Correct Answer: 15 years
Solution : Given: The compound interest of a certain sum becomes 2 times itself in 5 years. We know, $\text{Total Amount}=\text{Principal}×(1+\frac{\text{Rate}}{100})^{\text{Time}}$ According to the question, $2P=P[1+\frac{R}{100}]^5$ ⇒ $2=[1+\frac{R}{100}]^5$-------------(1) The number of years it becomes 8 times itself, So, $8P=P[1+\frac{R}{100}]^T$ ⇒ $2^3=[1+\frac{R}{100}]^T$----------------(2) From equation (1) and equation (2), we get, ⇒ $([1+\frac{R}{100}]^5)^3=[1+\frac{R}{100}]^T$ ⇒ $[1+\frac{R}{100}]^{15}=[1+\frac{R}{100}]^T$ ⇒ $T=15$ years Hence, the correct answer is 15 years.
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Question : If a sum triples in 4 years at simple interest, find the time that it would take to become 5 times itself at the same rate of interest.
Option 1: 6 years
Option 2: 8 years
Option 3: 5 years
Option 4: 3 years
Question : A certain sum of money amounts to 3 times itself in 13 years when interest is compounded annually at a certain interest rate per annum. In how many years will the initial sum amount to 9 times itself at the same interest rate per annum, also compounded annually?
Option 1: 32 years
Option 2: 26 years
Option 3: 30 years
Option 4: 20 years
Question : At a certain rate of interest per annum, compounded annually, a certain sum of money amounts to two times itself in 11 years. In how many years will the sum of money amount to four times itself at the previous rate of interest per annum, also compounded annually?
Option 1: 20 years
Option 2: 5.5 years
Option 3: 22 years
Option 4: 33 years
Question : A sum of money becomes 1.331 times in 3 years as compound interest. The rate of interest is:
Option 1: 8%
Option 2: 7.5%
Option 3: 10%
Option 4: 50%
Question : A sum of Rs. 3000 becomes Rs, 4320 in 2 years at a certain rate of compound interest (compounding annually). How much this sum will become after 4 years?
Option 1: Rs. 7568.50
Option 2: Rs. 6220.80
Option 3: Rs. 6516.80
Option 4: Rs. 5128.50
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