The simple and compound interests on a sum of money for $2$ years are ₹ $8400$ and ₹ $8652$ respectively. The rate of interest per annum is (in $\%$):

$A$ sum of money at compound interest doubles itself in $15$ years. It will become eight times of itself in (in $years$):

If the compound interest on a sum for $2$ years at $12 \frac{1}{2} \%$ per annum is ₹ $510$,the simple interest (in ₹) on the same sum at the same rate for the same period of time is

$A$ sum of money at compound interest amounts to thrice itself in $3$ years. In how many years will it be $9$ times itself?

An amount of money at compound interest grows up to ₹ $3,840$ in $4$ years and up to ₹ $3,936$ in $5$ years. Find the rate of interest (in $\%$).

If the difference between the simple and the compound interest earned on a sum of money at the rate of $5$ $p.c.p.a.$ for $2$ $years$ is $Rs. 16$,find the principal (in $Rs.$).