a)      What is the mean commute time for these 18 cities?   b)      What is the median commute time for these 18 cities?   c)      What is the mode for these 18 cities?

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a)

The mean commute time for these 18 cities is,

$\begin{array}{rcl}Mean& =& \frac{\sum _{}X}{n}\\ & =& \frac{24.9+26.8+26.2+28.1+31.7+25.8+23.3+32.1+28.5+24.4+29.3+24.8+38.1+28.3+28.4+24.6+20.1+23.4}{18}\\ & =& \frac{488.8}{18}\\ & =& 27.1556\end{array}$

Thus, the mean commute time for these 18 cities is 27.1556.

b)

The data in the ascending order is 20.1, 23.3, 23.4, 24.4, 24.6, 24.8, 24.9, 25.8, 26.2, 26.8, 28.1, 28.3, 28.4, 28.5, 29.3, 31.7, 32.1, and 38.1.

For even data set, the median is the average of ${\left(\frac{n}{2}\right)}^{th}$ and ${\left(\frac{n}{2}+1\right)}^{th}$ observation.

The sample size is 18, than the median is the average of 9^th temperature and 10^th temperature. The median commute time for these 18 cities is,

$\begin{array}{rcl}Median& =& \frac{26.2+26.8}{2}\\ & =& \frac{53}{2}\\ & =& 26.5\end{array}$

Thus, the median commute time for these 18 cities is 26.5.