Question

A student driving home for the holidays starts at 8:00 am to make the 675-km trip, practically all of which is on nonurban interstate highway. If she wants to arrive home no later than 3:00 pm, what must be her minimum average speed? Will she have to exceed the speed limit?

Answer 1

8 am to 3 pm is 7 hours.

(675 km) / (7 hrs) = 96.4 km/hr .

Her average speed for the whole 7 hours has to be not less than 96.4 km/hr. Any less, she's not home by 3:00.

We don't technically know the speed limit at every point on her trip, because you technically haven't told us.

But in 7 hours, she MUST stop for gas, she MUST get some rest, she MUST make a pit stop, and she most likely encounters some traffic somewhere. So in order to average 96.4 for the whole trip, she MUST exceed it for PART of the trip ... possibly by a lot.

Whatever the speed limit may be, I think it's likely that she'll exceed it SOMEwhere, at least for SOME time.