Simple Interest, Rate, time, Interest and time is same

In a certain time, a sum becomes 4 times at the rate of 5% per annum. At what rate of simple interest, the same sum becomes 8 times in the same duration?

Simple interest of some money for 2 years at 10% rate ofinterest per annum is Rs.1200 . What is the difference in compound interest and simple interest if the rate of interest,time and Principal amount remains the same?

If x, y, z are three sum of money such that y is the simple interest on x and z is the simple interest on y for the same time and at same rate of interest, then we have____