A sum of money on compound interest triples itself in 20 years. After 40 years, it will become :

9 times

We have,

A sum of money on compound interest triples itself in 20 years.

Means,

           Sum       Amount             Time

            1             3              ( In 20 years)

            1            3² = 9       ( In 40 years)

Method,

If the amount (a) becomes n time after p years then, after n × m time the amount will become a^m