Calculate the number of half lives elapsed, the end of which, the activity of a radioactive sample decrease by 90%.

Explanation

To solve the problem of calculating the number of half-lives elapsed for a radioactive sample to decrease by 90%, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Problem**: We need to find how many half-lives it takes for the activity of a radioactive sample to decrease by 90%. This means that if we start with 100% activity, we want to find out how long it takes for the activity to drop to 10%.

2. **Set Up the Equation**:
- Let the initial activity of the sample be \( A_0 = 100\% \).
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