Question : A sum of money placed at compound interest doubles itself in 5 years. In how many years would it amount to eight times itself at the same rate of interest?
Option 1: 10 years
Option 2: 15 years
Option 3: 7 years
Option 4: 20 years
Correct Answer: 15 years
Solution : By applying the formula: Amount = P[$(1+\frac{r}{100})^n$] where P is principal, $r$ is the rate of interest compounded annually for $n$ years. Let the sum be $x$, then: Amount = $x(1+\frac{r}{100})^5 = 2x$ ⇒ $(1+\frac{r}{100})^5 = \frac{2x}{x}=2$ ⇒ $(1+\frac{r}{100})=2^\frac{1}{5}$ Now, the sum becomes 8 times after $n$ years, then: $x(1+\frac{r}{100})^n = 8x$ ⇒ $(1+\frac{r}{100})^n = \frac{8x}{x}=8=2^3$ ⇒ $2^\frac{n}{5} = 2^3$ Thus, $n$ = 5 × 3 = 15 years Hence, the correct answer is 15 years.
Result | Eligibility | Application | Selection Process | Cutoff | Admit Card | Preparation Tips
Question : A certain sum doubles in 7 years at Simple Interest. The same sum under the same interest will become 4 times in how many years?
Question : A sum of money becomes double in 3 years at compound interest compounded annually. At the same rate, in how many years will it become four times of itself?
Question : At a certain rate of Simple Interest, a certain sum of money becomes double itself in 10 years. It will become triple of itself in:
Question : A sum amount doubles in 8 years by simple interest. Then the rate of simple interest per annum is:
Question : The simple interest on a sum of money at 10% per annum for 2 years is 8,100. Compounded annually, what would be the compound interest (in ) on the same sum for the same period at the same rate of interest?
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile