Explanation:

Let the sum be P and rate of interest be R %.

it is given that the sum becomes rs 8000 in 3 years .

=> amount = 9( 1+ r/100)^n

8000= p( 1+ r/100)^3.........(1)

and , the sum becomes rs 10000 in 6 years,

a= p(1+r/100)^n

10000=p(1+r/100)^6

10000/p= (1+r/100)^6......(2)

now, by squaring equation number (1 )

(8000)^2=p^2 ( 1+ r/100)^6

=> 64000000 = P^2 x 10000/p ( by using eq. no 2 )

p = 6400