At what simple rate of interest a sum is double in 8 years?

Rs. 2,000 is invested at 10% p.a. on simple interest. If that interest is added to the principal after every 20 years, the amount will become Rs. 14,000 after

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? 

What is the ratio of the simple interest earned on a certain amount at the rate of 21% per annum for 8 years to that earned on the same sum at the same rate for 21 years?  

If the difference of interest of two banks of Rs 500 is Rs 2.50, find the difference between their rates.

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? 

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

₹ 500 was invested at 12 % per annum simple interest and another certain sum of money invested at 10 % per annum simple interest . If the total interest on both the sum after 4 years is 480, the latter sum of money is