Let the amount invested by Bimal be ₹P.
\[ I_{\text{Amal}} = I_1 + I_2 = 960 + 609 = ₹1,569 \]
Bimal invests ₹P at 7.5% simple interest for 1 year.
\[ I_{\text{Bimal}} = P \times \frac{7.5}{100} \]
\[ P \times \frac{7.5}{100} = 1569 \] \[ P = \frac{1569 \times 100}{7.5} = ₹20920 \]
Final Answer: The amount invested by Bimal is \( \boxed{₹20920} \).
For any natural number $k$, let $a_k = 3^k$. The smallest natural number $m$ for which \[ (a_1)^1 \times (a_2)^2 \times \dots \times (a_{20})^{20} \;<\; a_{21} \times a_{22} \times \dots \times a_{20+m} \] is: