A sum of money was put at simple interest at a certain rate for 2 years. If this sum had been put at $3 \%$ higher rate, it would have earned ₹ 720 more as interest. Find the sum.
Naveen and Praveen borrowed $₹ 42000$ and $₹ 55000$ respectively for $3 \frac{1}{2}$ years at the same rate of interest. If Praveen has to pay ₹ 3640 more than Naveen, find the rate of interest.
Divide $₹ 6000$ into two parts so that the simple interest on the first part for 9 months at $12 \%$ per annum is equal to the simple interest on the second part for $1 \frac{1}{2}$ years at $10 \%$ per annum.