Question

A sum doubles in 20 years at simple interest. How much is the rate of interest per annum? ​

Answer 1

let the sum is x

amount is 2x

interest 2x-x=x

ATQ

$\frac{prt}{100} \\ \frac{x \times r \times 20}{100} = x \\ \frac{r}{5} = 1 \\ r = 5$

rate of interest is 5 % p.a