Travelling to some place and back again mightinvolve travelling twice the distance of only travelling there, butmight well not mean taking twice as long. Going in one directionmight mean travelling uphill or going against the wind or travellingin rush hour, meaning that this part of the total journey takeslonger.
To find a total time or the speed over one or bothparts of the journey can involve setting up an equation to be solved.
The distance from London toBrighton is 55 miles. The average speed from Brighton to London is
mph and the average speed from London to Brighton is
mph.The total travel time is 2 hours. Find
The Brighton to Londonjourney takes
hours
The London to Brightonjourney takes
hours.
The total journey takes 2hours so
We have to solve theequation
Wecan start by multiplying by 
toclear all the fractions, giving
or
mph.The second of these is obviously wrong so
mph.