If the simple interest for 6 years be equal to 30% of the principal, it will be equal to the principal after = ?

$$\eqalign{ & {\text{Let principal = 10P}} \cr & {\text{Interest = 10P}} \times \frac{{30}}{{100}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{ = 3P}} \cr & {\text{According to the question,}} \cr & {\text{Case (I)}} \cr & \Rightarrow 3{\text{P = }}\frac{{{\text{10P}} \times {\text{R}} \times {\text{6}}}}{{100}} \cr & \Rightarrow {\text{R = 5}}\% \cr & {\text{Case (II)}} \cr & {\text{Interest = Principal = 10P}} \cr & \Rightarrow {\text{10P = }}\frac{{{\text{10P}} \times {\text{5}} \times {\text{t}}}}{{100}} \cr & \Rightarrow {\text{t = 20 years}} \cr} $$