right  it is a hundred miles from king's lynn to london, the train takes two hours to do the journey  the train does not go at a constant speed, it speeds up sometimes and slows down at other times  it also stops at stations on the way  and on once of course as it, as it's stopping it's going more and more slowly and as it's  er moving off again it starts slowly and starts to go quickly but because it takes two hours in all  the train goes a hundred miles in two hours we say its average speed for the journey is fifty miles per hour.