Question

A Train travelling at 72 kmph crosses
another train moving at 108 kmph in
opposite direction in 9 seconds.
What is the length of first train if both
of them are of same length?
Select one:
a. 350 m
b. none of these
C. 450 m
d. 175 m​

Answer 1

Answer:

option b is correct. hope you will be brighter in future.

Answer 2

Given:

Speed of one train, s₁ = 72 km/hr = $72 \times \frac{5}{18}\:m/s = 20\:m/s$

Speed of another train, s₂ = 108 km/hr = $108 \times \frac{5}{18}\:m/s = 30\:m/s$

Time taken by both the trains to cross each other by travelling in opposite direction, t = 9 seconds

To find:

The length of first train if both  of them are of same length

Solution:

Since both the trains are of the same length, so we can assume the length of each train as "L₁" = "L₂" = "L" meter.

∴ Total length of both the trains = L + L = 2L meters

Also, the trains are moving in the opposite direction

∴ Relative speed = s₁ + s₂ = 20 + 30 = 50 m/s

So, to find the time we will use the following formula,

$Time \:=\: \frac{Total \:length\:of\: both\: the\: trains }{Relative \:Speed}$

Now, by substituting the values of time, total length and relative speed in the formula, we get

$9 = \frac{2L}{50}$

⇒ $2L = 9\times 50$

⇒ $L \:=\: \frac{450}{2}$

⇒ $L = 225 \:m$

∴ L₁ = L₂ = L = 225 m ← option (b): none of these

Thus, the length of the first train if both  of them are of the same length is 225 m.

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