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 = P(1+ r/100)ⁿ

8000= P(1+ r/100)³........(1)

and , the sum becomes Rs 10000 in 6 years,

10000 = P(1+r/100)⁶

10000/P= (1+r/100)⁶......(2)

now, by squaring equation number (1 )

(8000)² = P²( 1+ r/100)⁶

=> 64000000 = P² x 10000/P ( by using eq. no 2 )

P = Rs 6400