The given G.P. is

5

2

,

5

4

,

5

8

, ... . Let a be the first term and r be the common ratio of G.P.
Here a =

5

2

and r =

( 5 4 )

( 5 2 )

=

1

2

Then nth term is
an = arn−1 =

5

2

(

1

2

)n−1 = 5 (

1

2

)n =

5

2n

and 20th term is a20 =

5

220