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

In 5 years, the simple interest on an amount of Rs. X is $$2\over 5$$ of the principal. The rate of interest per annum is:

Manoj deposited ₹ 29400 for 6 years at a simple interest. He got ₹ 4200 as interest after 6 years. The annual rate of interest was-

There is 80% increase in an amount in 8 years at simple interest. What will be the compound interest on Rs. 15,000 after 3 years at the same rate of interest?

In what time will ₹ 72 become ₹ 81 at $$6{1\over4}$$ % per annum simple interest?

What will be the ratio of the interests earned by identical sum of money invested at the same rate of simple interest for 6 years and for 15 years respectively?

If $${8+2\sqrt{3} \over 3\sqrt{3}+5}=a\sqrt{3}-b$$, then the value of a+b is equal to:

The simple interest on ₹ 7,300 from 11 May, 1987 to 10 September, 1987 (both days included) at 5% per annum is-