To calculate the standard error (SE) of the mean, you can use the following formula:
\[ SE = \frac{\sigma}{\sqrt{n}} \]
Where:
- \(\sigma\) is the population standard deviation,
- \(n\) is the sample size.
In this case:
- \(\sigma = 20\) minutes,
- \(n = 100\).
Now, substituting the values into the formula:
\[ SE = \frac{20}{\sqrt{100}} = \frac{20}{10} = 2 \]
So, the standard error of the mean is 2 minutes.