Three samples of a radioactive substance have activity in a ratio 2 : 5 : 7 , then after two half lives the ratio of their activities will be:

Explanation

To solve the problem, we need to understand how the activity of radioactive substances changes over time, particularly after a certain number of half-lives.

### Step-by-Step Solution:

1. **Understanding Activity and Half-Life**:
The activity of a radioactive substance decreases over time. The activity \( A(t) \) at time \( t \) can be expressed as:
\[
A(t) = A_0 e^{-\lambda t}
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