Rs. 1000 doubles after 5 years at compound interest. What amount will it becomes after 20 years?

$amount is 2 times of principal in 5yrs$

$then amount becomes 2^{m } times in 5\times m years$

          $amount becomes 2^4 times in 5\times 4 years$

$so, in 20 years amount is 16 times of the principal$

i.e 16$\times 1000=16000$