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 of cobalt-60 after 4 years, we can follow these steps:

### Step 1: Understand the half-life
The half-life (T_half) of cobalt-60 is given as 5.26 years. This means that after 5.26 years, half of the original amount of cobalt-60 will remain.

### Step 2: Determine the decay constant (k)
The decay constant (k) can be calculated using the formula:
\[ k = \frac{0.693}{T_{half}} \]
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