Find the least number of complete years in which a sum of money put out at $20 \%$ compound interest will be more than double?

The difference between the simple and compound interest on a certain sum of money for $2$ years at $4 \%$ per annum is $Rs. 4$. Find the sum (In $Rs.$)?

Abhinav invested a certain amount at the rate of $8$ $p.c.p.a.$ for $5$ years and obtained a $SI$ of $Rs. 3800$. Had he invested the same amount at the same interest rate for $2$ years,how much amount (in $Rs.$) would he have obtained as $CI$ at the end of $2$ years?

If the ratio of $C.I.$ and $S.I.$ interest for the same principal and rate of interest for $2$ years is $1.2,$ find the rate of interest (in $\%$).

$A$ 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

$A$ sum of money doubles in $3$ years at compound interest,compounded annually. It will become $4$ times of itself in: (in $years$)