I think the motorcycles are traveling twice as fast as the moped. Not sure how to prove this. I tried one case, assuming the hill was 18 miles long so that the moped took 60 minutes to reach the top. Then, assuming the motorcycles traveled at twice the speed, they would reach the top in 30 minutes. So if one leaves the starting point every 30 minutes then 30 will pass the moped when it reaches the top (the 30th will reach the top just as the moped does). When the moped goes back down at 36 mph it will reach the bottom in 30 minutes, and in those 30 minutes it will pass 30 motorcycles. QED (sort of).