s = speed of snowmobile (mph)

d = speed of dog sled (mph)

Snowmobile is 40mph faster than dog sled:  s = d + 40

A dog sled travels 24 miles in the same time that the snowmobile travels 104 miles:

24/d = 104/s  =>  d/24 = s/104  =>  d = 24s/104  =>  d = 3s/13

Substitute d = 3s/13 into the first equation and solve:  s = (3s/13) + 40

=>  13s = 3s + 520  =>  10s = 520  =>  s = 52

Now plug in 52 for s and solve:  (52) = d + 40  =>  d = 12

So the snowmobile travels at 52mph and the dog sled travels at 12mph.

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