Gopal invests a sum of ₹5,400 and Akshay invests a sum of ₹10,200 at the same rate of simple interest per annum. If, at the end of 4 years, Akshay gets ₹720 more interest than Gopal, then find the rate of interest per annum (in percentage).
When difference between compound and simple interest for three years is ₹93 at 10% interest per annum, the principal is ₹______
The cost of painting a spherical vessel of radius 7 cm is ₹18,480. What is the cost of painting per square centimetre? (Use π = \( \frac{22}{7} \))