To solve the problem:

The formula for calculating CFU/mL is:

$\text{CFU/mL} = \frac{\text{Number of Colonies}}{\text{Volume Plated (mL)}} \times \text{Dilution Factor}$

Given:

• Number of colonies = 300
• Volume plated = $0.120 \, \text{mL}$
• Dilution factor = $1 \times 10^{-1} = 0.1$

Now substitute into the formula:

$\text{CFU/mL} = \frac{300}{0.120} \times 10$

Step-by-step:

1. Divide 300 by 0.120: $\frac{300}{0.120} = 2500$
2. Multiply by 10: $2500 \times 10 = 25,000 \, \text{CFU/mL}$

Answer: 25,000 CFU/mL

Let me know if you want more clarification or have any questions!

Additional Questions:

1. How does the dilution factor influence CFU/mL calculations?
2. What is the importance of plating a consistent volume in such calculations?
3. How would the CFU/mL change if only 150 colonies were counted?
4. What is the significance of CFU/mL in microbiology?
5. Why is it necessary to multiply by the inverse of the dilution factor?

Tip: Always ensure you include the proper dilution factor when calculating CFU/mL to avoid errors in your final result.