Simple Interest formula: \( SI = \frac{P \times R \times T}{100} \)
Here, \( P = 20,000 \), \( R = 13% \), \( T = \frac{42}{12} = 3.5 \) years
\[
SI = \frac{20000 \times 13 \times 3.5}{100} = \frac{910000}{100} = 9100
\]
\[
\text{Total Amount} = 20000 + 9100 = \boxed{\rupee 29,100}
\]