Four vehicles travelled on a road. What can you deduce from the times that they met?
Brian swims at twice the speed that a river is flowing, downstream
from one moored boat to another and back again, taking 12 minutes
altogether. How long would it have taken him in still water?
At Holborn underground station there is a very long escalator. Two
people are in a hurry and so climb the escalator as it is moving
upwards, thus adding their speed to that of the moving steps. ...
How many steps are there on the escalator?
The reason the average of the average speeds isn't the overall
average speed is because the journey time isn't the same there and
back . . . . but the distance is, and that's useful if instead I
use the reciprocal of the rate.
Speed is a rate (the rate at which distance changes as time
passes) and any rate can be considered in it's reciprocal form (the
rate at which time passes per unit distance)
The distance is the same so adding the reciprocal of the speeds
and then dividing by two does give me the reciprocal of the overall
and this gives the answer 75 mph