I assumed that the hill was empty when the moped started down. But there will be motorcycles already on the hill, plus those that will start up as the moped goes down.
I think the answer to the question is that the motorcycles travel at 4 times the speed of the moped (72 mph).
My equation that solves this is
(((D/M)x60) - ((D/S)x60)))=(((D/2M)x60 + ((D/S)x60)))
where D is the distance the moped travels,
M is the speed the moped travels,
S is the speed the motorcycles travel.
The left side is the number of motorcycles that pass the moped going up, the right side is the number of motorcycles that the moped passes going down. The '60' converts mph to mp minute.