To find the average rate of change of the function \( f(x) = 0.01(2)^x \) from day 3 to day 8, we can use the formula for the average rate of change over an interval \([a, b]\):
\[ \text{Average Rate of Change} = \frac{f(b) - f(a)}{b - a} \]
Here, we have \( a = 3 \) and \( b = 8 \).
First, we calculate \( f(3) \):
\[ f(3) = 0.01(2)^3 = 0.01 \times 8 = 0.08 \]
Next, we calculate \( f(8) \):
\[ f(8) = 0.01(2)^8 = 0.01 \times 256 = 2.56 \]
Now we substitute these values into the average rate of change formula:
\[ \text{Average Rate of Change} = \frac{f(8) - f(3)}{8 - 3} = \frac{2.56 - 0.08}{5} = \frac{2.48}{5} = 0.496 \]
Thus, the approximate average rate of change from day 3 to day 8 is \( \boxed{0.496} \).