Nucleus: Practice Problem & Solution

Solution Explained:

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

Remaining nuclei at $t_1$ is $100 - 40 = 60\%$. Remaining at $t_2$ is $100 - 85 = 15\%$. The ratio is $15/60 = 1/4 = (1/2)^2$, meaning 2 half-lives have passed. Time $= 2 \times 30 = 60$ minutes.