well, if he travelled 60 miles in 45 mins, it means his average speed was 80 mph. therefore, at some point he must have been going at 80 mph for it to be the average. thus he exceeded 55 mph at some point. if you know the mean value theorem you canuse that to prove it elegantly--that's calculus though (i think, i never learnt it till calculus), so if you're not in calculus, forget about it