Simple interest
a) the interest on $18,000 invested for 10 years is $12,000. what is the rate per cent?
b) in another case, what principal is needed so that the interest will be $4,800 if it is invested at 3% per annum for 5 years?
c) provident bank offers a 3-year cd that earns 3.64% per annum. an investor collected an amount of $11,092 at the end of 3 years. how much did he invest in the cd?