The selling rate of a radioactive isotope is determined by its...

The selling rate of the radioactive isotope is directly proportional to its activity.

The activity of a radioactive sample after time $t$ is given by $A = A_0 \left(\dfrac{1}{2}\right)^{\dfrac{t}{t_{1/2}}}$.

Since the price is proportional to the activity, the new price $P$ after time $t$ is $P = P_0 \left(\dfrac{1}{2}\right)^{\dfrac{t}{t_{1/2}}}$.

Given $P_0 = 800 \text{ rupees}$, $t = 30 \text{ days}$, and $t_{1/2} = 14.3 \text{ days}$.

Substituting the values:

$P = 800 \left(\dfrac{1}{2}\right)^{\dfrac{30}{14.3}}$

$P = 800 \left(\dfrac{1}{2}\right)^{2.098}$

$P = \dfrac{800}{2^{2.098}} \approx \dfrac{800}{4.28} \approx 187 \text{ rupees}$.