To express the growth rate as a percentage, we need to consider the initial number of infected individuals and the daily increase.

Given:

• Initial number of infected individuals after 3 days: $I(3) = 21$
• Daily increase (growth rate): $8$ people per day

The daily growth rate as a percentage can be calculated using the formula:

$\text{Growth Rate (\%)} = \left( \frac{\text{Daily Increase}}{\text{Initial Amount}} \right) \times 100$

Substituting the given values:

$\text{Growth Rate (\%)} = \left( \frac{8}{21} \right) \times 100$

Let's compute this:

$\text{Growth Rate (\%)} = \left( \frac{8}{21} \right) \times 100 \approx 38.0952$

Rounding to two decimal places:

$\text{Growth Rate (\%)} \approx 38.10\%$

So, the growth rate is approximately $38.10\%$ per day.

Would you like any details or have any questions about this problem?

Here are 5 more questions you might be interested in:

1. How do you convert a decimal growth rate to a percentage?
2. What is the difference between linear growth and exponential growth?
3. How can you calculate the growth rate over different intervals of time?
4. Why is it important to round percentages in real-world contexts?
5. How can understanding growth rates help in managing epidemics or other real-world scenarios?

Tip: When converting between decimals and percentages, always multiply by 100 and remember to adjust your rounding based on the context or instructions provided.