In a certain number of years,a sum of money doubles itself at $6 \frac{1}{4} \%$ simple interest per annum. The required time (in $years$) is:

What sum of money (in $Rs.$) at compound interest will amount to $Rs. 5305.53$ in $3$ years,if the rate of interest is $1 \%$ for the first year,$2 \%$ for the second year,and $3 \%$ for the third year?

$A$ tree increases annually by $\frac{1}{8}$ of its height. By how much will it increase (in $cm$) after $2$ years,if it stands today $64 \ cm$ high?

$A$ bank offers $5 \%$ compound interest calculated on a half-yearly basis. $A$ customer deposits ₹ $1600$ each on $1^{st}$ January and $1^{st}$ July of a year. At the end of the year,the amount (in ₹) he would have gained by way of interest is

$A$ certain sum at compound interest amounts in two years to ₹ $2809$ and in three years to ₹ $2977.54$. Find the rate of interest and the original sum (in ₹).

The compound interest (in ₹) on ₹ $12000$ for $9$ months at $20 \%$ per annum,interest being compounded quarterly,is