The interest for 6 years is 30% of the principal, meaning the rate of interest per year is $\frac{30}{6}=5\mathrm{\%}\backslash frac\{30\}\{6\}\; =\; 5\backslash \%$.
To find when the interest equals the principal, we use the formula $Interest=P\times R\times T\backslash text\{Interest\}\; =\; P\; \backslash times\; R\; \backslash times\; T$Interest=P×R×T, where R=5%.
For interest to be equal to the principal, $100\mathrm{\%}100\backslash \%$ interest is needed, and at 5% per year, this will take $\frac{100}{5}$​=20 years.