Two friends were driving from their home to Manchester, Kevin drove the first 90 miles, and Daniel took over the remainder of the journey.
[Ref: ZIMH] © Kevin Stone
On the way back, Kevin drove to begin with, and Daniel took over for the last 100 miles.
Who drove the most?
On the way there Kevin drove 90 miles, and on the way back drove the (total distance - 100 miles).
On the way Daniel drove the (total distance - 90 miles), and on the way back drove 100 miles.
So Kevin drove 90 + total - 100 = total - 10.
And Daniel drove total - 90 + 100 = total + 10.
So Daniel drove 20 miles more than Kevin. QED.
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