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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:
Answer & Solution
Correct Answer:
Option
C
1st 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
2nd Method:
As in both cases, the speed of the train is constant, and then we have;
Time α distance
$$\eqalign{
& \frac{{100}}{{60}} = \frac{{x + 800}}{{x + 400}} \cr
& {\text{or,}}\,\,x = 200{\text{m}} \cr} $$
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Login=>400m=40s......(1)
In question it is given that train crossing 400m length bridge in 60s,but from eq(1) we get that train cross 400m in 40s,that means train takes 40 second to cross the length of bridge.in another 20second it must be crossing its own length.
So;
=>40 s=400 m
=>(40s/2=20s)=400m/2=200m
So length of train is 200 m. (Ans.)
Or, 5x+200=3x+2400
It will be not 200 after 5x,it will be 2000...