If a sum doubles in 12 years, how much will it be in 8 years ?

Question: If a sum doubles in 12 years, how much will it be in 8 years ?

Solution:
let the rate is r
we know,
I = Pnr/100
r = (I × 100)/Pn
= (P × 100)/(P × 12)
= 100/12 %

in 8 years,
I = Pnr/100
= (P × 8 × 100/12)/100
= 2P/3
= 2/3 of P

It will be (1 + 2/3) = 5/3P