What is simple interest

At what rate of simple interest a certain sum will be doubled in 12 years ?

What is the simple interest of Rs. 1550 at the rate of 4% in 39 months ?

At what rate of simple interest a certain sum will be doubled in 15 years?

At what rate of simple interest a certain sum will be doubled in 14 years

At what rate of simple interest per annum in percent the interest of a principal in 20 years will be (4)/(9) part of its amount ?

At what rate of simple interest in percent per annum, the ratio of principal and its interest after 20 years is 1:1 ?

If an amount of Rs 5,000 earned a simple interest of Rs 500 over 2years, what is the rate of simple interest?