Why is it important to use 0, representing those who said they did not work at all, in finding an average?

Thirty-six percent of US high school seniors in the survey did not work at all, while 21% of them worked more than 21 hours per week.

Is it possible for two very different sets of data to have the same mean?

If there are one or more unusually large numbers with respect to the rest of the data in a data set, the mean may be much larger than seems typical. If there are one or more unusually small numbers in the set with respect to the rest of the data, the mean may be much smaller than seems typical.

Why is it reasonable to use 25 as an upper bound for the 21+ category in the data of the challenge?