Question

On a journey of 600 km, a train was delayed 1 hour and 30 min after having covered 1/4 of the way. To arrive on time at the destination, the engine driver had to increase the speed by 15 km/hour. How long did the train travel?

Answer 1

10 hours

Explanation:

(150/x) + 1.5 + (450/(x+15)) = 600/x

Multiply through by x(x + 15):

150(x + 15) + 1.5x(x + 15) + 450x = 600(x + 15)

150x + 2250 + 1.5x^2 + 22.5x + 450x = 600x + 9000

150x + 2250 + 1.5x^2 + 22.5x + 450x - 600x - 9000 = 0

1.5x^2 + 22.5x - 6750 = 0

x = (-22.5 +/- sqrt(22.5^2 - 4(1.5)(-6750))) / (2*1.5)

x = (-22.5 +/- sqrt(506.25 + 40500)) / 3

x = (-22.5 +/- sqrt(41006.25)) / 3

x = (-22.5 +/- 202.5) / 3

x = 180/3 or -225/3

x = 60 or -75

But a negative number doesn't make sense, so therefore x = 60, so the journey took 600/60 = 10 hours.