a train takes 2 hours less for a journey of 300 km if its speed is increased by 5 km/h from normal speed. find the normal speed
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time taken =distance / speed
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Let x = normal speed
300 / (x + 5) + 2 = 300 / x
. . . combine left side into a single fraction
(310 + 2x) / (x + 5) = 300 / x
. . . cross multiply
310x + 2x^2 = 300x + 1500
2x^2 + 10x - 1500 = 0
x^2 + 5x - 750 = 0
(x - 25) (x + 30) = 0
x = -30 <== invalid negative time
x = 25 km / h <===== answer
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