Arun gives his wife Asha a birthday gift,giving her each year a number of rupees equal to the number of years of her age. If her birthday falls on $August \, 8$,what sum must be placed at simple interest at $7 \%$ on $January \, 1$,before she is $42$,in order to raise the required sum (in $Rs.$)?

At what percent of simple interest will a sum of money double itself in $15$ years?

Alipta received some amount of money from her father. In how many $years$ will the ratio of the money (principal) and the interest obtained from it be $10: 3$ at the rate of $6 \%$ simple interest per annum?

In what time (in $years$) will ₹ $1200$ become ₹ $1344$ at $6 \%$ per annum simple interest?

Sapna borrowed a certain sum of money from Kavita under the following repayment scheme based on simple interest: $8 \%$ $p.a.$ for the initial $2$ $years$,$9.5 \%$ $p.a.$ for the next $4$ $years$,$11 \%$ $p.a.$ for the next $2$ $years$,and $12 \%$ $p.a.$ after the first $8$ $years$. Find the total amount (in $Rs.$) which a sum of $Rs. 9000$ taken for $12$ $years$ becomes at the end of $12$ $years$.

If the simple interest on a certain sum of money is ₹ $256$ and the rate of interest per annum equals the number of years,then the rate of interest (in $\%$) is: