During a recent BrainBashers race meeting, the fastest driver managed to complete a lap with an average speed of 150 mph.
They managed to complete the first two fifths of the lap at a speed of 123 mph and the second two fifths of the lap at a speed of 164 mph.
At what speed was the final one fifth of the lap length covered?
The answer is not 176 mph.
Puzzle Copyright © Kevin Stone
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Hint
Adding the speeds won't work.
Answer
205 mph.
Reasoning
If we take the number of miles in a lap to be L miles and use Time = Distance ÷ Speed.
The first 2/5 are covered at 123 mph. Therefore, the time taken was [(2/5) x L] ÷ 123 hours.
The next 2/5 are covered at 164 mph. Therefore, the time taken was [(2/5) x L] ÷ 164 hours.
The last 1/5 is covered at y mph. Therefore, the time taken was [(1/5) x L] ÷ y hours.
The entire trip is covered at an average of 150 mph, and we can conclude that:
2L 2L L L
————— + ————— + ——— = ———
5x123 5x164 5y 150
Rearranging and simplifying gives:
1 1 2 2
——— = —— - ——— - ———
y 30 123 164
That means that y = 1 ÷ (1/30 - 2/123 - 2/164)
Using a calculator gives y = 205.
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