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

The simple interest for four years is one fourth of the principal. At the same rate, which amount become Rs.450 total amount in 2 years?

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?  

A person lends 40 % of his sum of money at 15 % per annum, 50 % of rest at 10 % per annum and the rest at 18 % per annum rate of interest. What would be the annual rate of interest, if the interest is calculated on the whole sum? 

The difference between C.I and S.I for 2 years is Rs. 9. Find the sum (in Rs.) if the rate is 15% per annum.

A sum of ₹ 7, 930 is divided into 3 parts and given at loan at 5 % simple interest to A, B and C for 2, 3 and 4 years respectively. If the amounts of all three are equal after their respective periods of loan, then the A received a loan of 

The simple interest of 7 years of money is Rs. 1750. If the rate of interest was 2% more per annum, how much interest would have been higher?

What would be the simple interest obtained on amount of Rs.6,535 at the rate of 10% Percent per annum after 6 years?