Nucleus: Practice Problem & Solution

Short Solution (Best for NEET):

To solve this problem, we apply the core principles of Nucleus. Understanding the underlying formula is key to arriving at the correct answer below:

Time elapsed is 2 hours = 120 minutes. This is $120/30 = 4$ half-lives. The final count rate is $A = A_0 / 2^4 = A_0 / 16$. Given $A = 5 second^{-1}$, the initial count rate $A_0 = 5 \times 16 = 80 second^{-1}$.