I want to travel 1,000 miles on an adult tricycle and I have two spare tyres.
However, I want to use all five tyres equally and have an equal mileage on each of the five tyres by the end of the trip.
Of course I can only use three tyres at a time, so I will have to change tyres during the trip.
At the end of the trip how many miles will each tyre have been used?
A total of 1,000 miles have been travelled and at any one time, three tyres are travelling. Therefore 3,000 tyre miles have been travelled, which is shared equally by the 5 tyres. QED.
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