Let Principal be P
SI after 6 years is 0.3P (30% of Principal)
Using SI formula to find rate of Interest
SI=PxRxT/100
0.3P=PxRx6/100
R=5%
Now R=5%, Principal =P and SI=P(Given that SI is equal to Principal)
SI=PxRxT/100
P=Px5xT/100
T=20
Hence after 20 years SI is equal to principal