A train with a speed of 90 km/h crosses a bridge in 36 seconds.

Question

A train with a speed of 90 km/h crosses a bridge in 36 seconds. Another train 100 m shorter crosses the same bridge at 45 km/h. What is the time taken by the second train to cross the bridge?

(A) 32 sec
(B) 40 sec
(C) 50 sec
(D) 64 sec
(E) 72 sec

Aryaa03 · Answer

IMO-Option-D

Length of 1st Train (m) = L1
Length of 2nd Train = L1-100
Length of Bridge (m) = L2
Speed of 1st Train = S1 = 90 km/hr = 90*1000/3600 = 25 m/sec
Speed of 2nd Train = 45 km/hr = 45*1000/3600  = 12.5 m/sec

For first train to cross the entire bridge, distance to be covered = L1 + L2 at a speed of 25 m/sec
==> (L1+L2)/25 = 36
==> L1+L2 = 900 m  --(1)

As the length of 2nd train is 100 m less than that of 1st, for 2nd Train, time required to cross the bridge,
Time required, t = (L1-100+L2)/12.5
subsitute value of L1+L2 from 1st equation as 900
==>t =  (900-100)/12.5 = (800*2)/25 =  64 seconds

For second Train

Themightyknight · Answer

Instead of solving 
I went with assumption
Train A - 90km/hr time 36sec
Train B - 45km/hr time 72sec

But train is 100 Mt shorter of a but less time this 64sec

Posted from my mobile device

Abhishek009 · Answer

90  kmph  = 90*\frac{5}{18} = 25  mps
 45  kmpmh  = 45*\frac{5}{18} = \frac{225}{18} = 12.5  mps

T + B = 36*25 
Or,  T + B =  900

\frac{(T - 100 + B)}{12.5} = ?

We know,  T + B = 900

So,  \frac{(900 - 100)}{12.5} = 64 , Answer must be (D)

PSKhore · Answer

Convert both the speeds into m/s
90*(5/18) = 25 m/s

The other one is 45, so it will be half of this = 12.5 m/s

Now, distance = speed*time
25*36 = 900 meters

The other distance is 100 less, so it will be 800 meters.
speed = distance/time
12.5 = 800/time
time =  64 sec

OR

By observation, if speed is halved, time taken will double, so for 100 meters less, it should be a little less than 72 (36*2), so  64

A train with a speed of 90 km/h crosses a bridge in 36 seconds.

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­A train with a speed of 90 km/h crosses a bridge in 36 seconds.

Prep Toolkit

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My Rewards

A train with a speed of 90 km/h crosses a bridge in 36 seconds.