(b) 20 years.
Let the principal = Rs x and rate of interest = R% p.a.
Then, S.I. = 30% of Rs x = \(\frac{30}{100}\times x\)
∴ \(\frac{x\times{R}\times6}{100} = \) \(\frac{30}{100}\times x\)
⇒ R = \(\frac{30}{6}=\) 5 % p.a.
Let the time in which the principal is equal to simple interest be t years, then
\(\frac{x\times{5}\times{t}}{100} = x\) ⇒ t = \(\frac{100}{5}years\) = 20 years.