Question

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

Answer 1

let the sum be 100.100. 
Then double the sum means, 200.200.  

From the above we can say that for 2020 years the amount is 200200 and the interest is 100100 for 2020 years. for one year the interest is 5.5. 

We can easily say that, 55 is 5%5% of 100.100.
Therefore rate of interest is 5%5%

or

let sum=100,=100,

then using given data, for 2020 years,
amount = sum + interest =22(sum)=200=200

therefore interest is 100100 for 2020 years. 
For one year it is 55
Then interest =5100×100=5%

Answer 2

I think at the rate of 5 %