If an amount doubles automatically in 8 years, then what is the interest rate?

The simple interest on a certain sum of the money at 4 % per annum for 4 years is Rs.80 more than the interest on the same sum for 3 years at 5 % per annum. Find the sum.

2 years, a sum was put at SI at a certain rate. If the rate was 3 % higher, it would have fetched 300 more. What will be the sum? 

A sum of money at simple interest amount to Rs.4800 in 3 years and Rs.5600 in the 5 years. The sum is

The simple interest on a sum of money for 3 years is ₹240 and the compound interest on the same sum, at the same rate  for 2 years is ₹170. The rate of interest is 

The simple interest and the true discount on a certain sum for a given period and at a given rate are 25 and 20 respectively. The sum is 

In a certain time, the ratio of a certain principle and interest obtained from it are in the ratio 10 : 3 at 10 % interest per annum. The number of years for which the money was invested is? 

An amount doubles itself in 5 years simple interest. What is the rate of interest per year?