Question

At what rate of simple interest will a sum of money double inself in 20 years?

Answer 1

let sum=100
then using given data, for 
20 years,
amount = sum + interest =2
=2(sum)
=200
=200

therefore interest is 
100
100 for 
20
20 years. 
For one year it is 
5

Then interest =5×100÷100

5/100×100=5%