Final answer:
To find the median number of fire hydrants, arrange the values in ascending order and find the middle value.
Step-by-step explanation:
To find the median number of fire hydrants, we need to arrange the given numbers in ascending order: 8, 12, 15, 17, 21. As there are an odd number of values, the median will be the middle value.
Therefore, the median number of fire hydrants is 15.