Part A)
His sample only involves days during the summer (if Devin is in the northern hemisphere), which would mean that the high temperatures he'll record are likely to be higher than the average. Therefore, the sample mean he computes does not represent the average high temperature for the whole year.
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Part B)
To correct his mistake, he needs to sample every day of the year. Or he could sample a few days of each month (say the first ten days of each month). That way the entire year is better represented. The other seasons of spring, fall and winter are included now.
