If a sum of money doubles itself in $8$ years at $S.I.$,then the rate of interest per annum (in $\%$) is: (in $.5$)

Nutan invests $Rs. 22400$ at a Simple Interest $(S.I.)$ rate of $12$ $p.c.p.a.$ How much amount will she receive after $7$ years (in $Rs.$)?

$A$ man invested $\frac{1}{3}$ of his capital at $7 \%$,$\frac{1}{4}$ at $8 \%$,and the remaining at $10 \%$ rate of simple interest. If his annual income from interests is ₹ $561$,then the total capital invested (in ₹) was:

$A$ person lent ₹ $10000$ to $B$ for $3$ $years$ and ₹ $6000$ to $C$ for $4$ $years$ on simple interest at the same rate of interest and received ₹ $5400$ in all from both of them as interest. What is the rate of interest (in $\%$)?

During the first year,the population of a village is increased by $5 \%$ and in the second year it is diminished by $5 \%$. At the end of the second year,its population was $47880$. What was the population at the beginning of the first year?

If a sum of money at simple interest doubles itself in $6$ years,it will become four times in how many years?