A radioactive sample has `6.0 xx 10^18` active nuclei at a certain instant. How many of these nuclei will still be in the same active state after two half-lives?

Explanation

To solve the problem, we need to understand the concept of half-lives in radioactive decay. A half-life is the time required for half of the radioactive nuclei in a sample to decay.

### Step-by-Step Solution:

1. **Identify the initial number of active nuclei**:
The problem states that there are `6.0 x 10^18` active nuclei at the start.

2. **Calculate the number of nuclei after the first half-life**:
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