Let everybody chime in on this one and will see how many DIFFERENT solution can be had!!.
Here is my take:
The man travels 1 mile in 1/40 of an hour going, and in 1/60 of an hour coming:
Therefore: 1/2 [1/40 + 1/60] =1/48 of an hour, going and coming.
Therefore he will average the reciprocal of that, or 48 hours.