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

A sum of money amounts to ₹ $6655$ at the rate of $10 \%$ compounded annually for $3$ years. The sum of money is (In ₹) 

How many statements are useful for find this : 

What is the rate of interest percent $p.a.$?

$Statements:$

$I$. The difference between the compound interest and simple interest earned in two years on the amount invested is ₹ $100$.

$II$. The amount becomes ₹ $19,500$ in three years on simple interest.

$III$.The simple interest accrued in two years on the same amount at the same rate of interest is ₹ $3,000$.

The principal that amounts to ₹ $4913$ in $3$ years at $6 \frac{1}{4} \%$ per annum compound interest compounded annually (In ₹) is.

What is $C.I.$ (In $Rs.$) on $Rs. 7500$ for $4$ years if the rate of interest is $10 \%$ p.a. for the first $2$ year and $20 \%$ $p.a.$ for the next $2$ year?

What would be the $C.I.$ on $Rs. 17500$ at the rate of $12$ $p.c.p.a.$ after $2$ years (In $Rs.$) ?