The half-life of cobalt-60 is 5.26 years. Calculate the percentage activity remaining after 4 years.

Explanation

To solve the problem of calculating the percentage activity remaining for cobalt-60 after 4 years, we will follow these steps:

### Step 1: Understand the half-life concept
The half-life of a radioactive substance is the time required for half of the radioactive atoms in a sample to decay. For cobalt-60, the half-life is given as 5.26 years.

### Step 2: Calculate the decay constant (λ)
The decay constant (λ) can be calculated using the formula:
\[
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