$A$ certain sum of money amounts to $Rs. 756$ in $2$ $years$ and to $Rs. 873$ in $3.5$ $years$. Find the rate of interest (in $\%$)?

Prabhat took a certain amount as a loan from a bank at the rate of $8 \%$ simple interest per annum and gave the same amount to Ashish as a loan at the rate of $12 \%$ per annum on simple interest. If at the end of $12$ years,he made a profit of $Rs. 320$ in the deal,what was the original amount (in $Rs.$)?

$A$ certain sum becomes ₹ $1020$ in $5$ years and ₹ $1200$ in $8$ years at simple interest. What is the value of Principal (in ₹)?

Divide ₹ $2379$ into $3$ parts so that their amounts after $2, 3$ and $4$ years respectively may be equal,the rate of interest being $5 \%$ per annum at simple interest. The first part is (in ₹):

The population of a particular area of a city is $5000$. It increases by $10 \%$ in $1^{st} \,yr$. It decreases by $20 \%$ in the $2^{nd} \,yr$ because of some reason. In the $3^{rd} \,yr$ the population increases by $30 \%$. What will be the population of the area at the end of $3$ $years$?

Bindu lends $Rs. 10000$ in four parts to her friends. If she gets $8 \%$ on $Rs. 2000$,$7.5 \%$ on $Rs. 4000$ and $8.5 \%$ on $Rs. 1400$,what percent must she get for the remainder,if the average interest is $8.13 \%$?