$A$ sum of money put at simple interest trebles itself in $15$ years. The rate per cent per annum is (in $\%$):

$A$ money lender lends ₹ $400$ for $3$ $years$ to a person and ₹ $500$ for $4$ $years$ to another person at the same rate of simple interest. If altogether he receives ₹ $160$ as interest,what is the rate of interest per annum (in $\%$)?

At what rate per cent of simple interest will a sum of money double itself in $12$ years?

If a certain sum of money borrowed at $5 \%$ per annum simple interest amounts to ₹ $1020$ in $4$ years,then the sum of money (in ₹) borrowed is:

Two equal sums were lent at simple interest for $4$ years and for $3$ years respectively. The rate of interest in the latter case was $3 \%$ higher than that of the former case,but the amount in each was $Rs. 1088$. Find the rate of interest (in $\%$) of the former case?

Manish borrowed a sum of $Rs. 1150$ from Anil at the simple rate of $6$ $p.c.p.a.$ for $3$ years. He then added some more money to the borrowed sum and lent it to Sunil for the same time at $9$ $p.c.p.a.$ at simple interest. If Manish gains $Rs. 274.95$ by way of interest on borrowed sum as well as his own amount from the whole transaction,then what is the sum (in $Rs.$) lent by him to Sunil?