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1. A train passes two bridges of length 800 m and 400 m in 100 seconds and 60 seconds respectively. The length of the train is:

A. 80m

B. 90m

C. 200m

D. 150m

Solution:

1^st Method:
Let length of the train be x m and speed of the train is s kmph.
Speed, s =

\frac{x + 800}{100}

. . . . . (i)
Speed, s =

\frac{x + 400}{60}

. . . . . (ii)
Equating equation (i) and (ii), we get,
Or,

\frac{x + 800}{100}

\frac{x + 400}{60}

Or, 5x + 200 = 3x + 2400
Or, 2x = 400
Or, x = 200m

2^nd Method:
As in both cases, the speed of the train is constant, and then we have;
Time α distance

\begin{aligned} \frac{100}{60} = \frac{x + 800}{x + 400} \\ or, x = 200 m \end{aligned}

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