$A$ certain sum doubles in $7$ years at simple interest. The same sum under the same interest rate will become $4$ times in how many years?

$A$ sum of money at simple interest becomes four times in $24$ years. The rate per cent of interest per annum is

$A$ certain sum is invested for $T$ years. It amounts to ₹ $400$ at $10 \%$ $p.a.$. But,when invested at $4 \%$ $p.a.$,it amounts to ₹ $200$. Find the time (in years) $(T)$.

When the rate of interest in a bank is reduced from $5 \%$ p.a. to $4.5 \%$ p.a.,Bani deposited $Rs. 4000$ in her savings account. She found that the new interest income remained the same as before. Find her original deposit (in $Rs.$)?

Adam borrowed some money at the rate of $6 \% \text{ p.a.}$ for the first two years,at the rate of $9 \% \text{ p.a.}$ for the next three years,and at the rate of $14 \% \text{ p.a.}$ for the period beyond five years. If he pays a total interest of ₹ $11,400$ at the end of nine years,how much money (in ₹) did he borrow?

Rashmi lent $Rs. 600$ to Geeta for $2 \,yr$ and $Rs. 150$ to Seeta for $4 \,yr$ at the same rate of interest and received altogether $Rs. 80$ as simple interest from both. Find the rate of interest (in $\%$).