24 miles is the right answer.
Here is one way to work this (thanks originally to rhyme for the style of operation), while it may seem a little long, it's pretty fast and accurate once you get used to it.
Use d = st, let's ignore the 4 hours of rest because it does not count. The real travel time is 11 hours
d x x
s 8 3
t 11-t t
since distance is equal,
8(11-t) = 3t => 88-8t = 3t => 88 = 11t => t = 8
d 24 24
s 8 3
t 3 8
Distance between cities is 24.
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