Aptitude - Problems on Trains - Discussion

Let,

The speed of slower train be x m/sec.
The speed of faster train be 2x m/s.

Now,
(x+2x)÷2(two trains) = 100/8.
or,x=25/3.

The Speed of faster train = 2x = 2 * 25/3 = 50/3 m/sec * 18/5 = 60km/hr.

Why the relative speed is the sum of individual speeds rather than the difference?

Let;
Speed of slower train (u) = X m/sec.
Speed of faster train(v) = 2X m/sec.

Because the second train is twice as faster the first train.
Each train is 100 m long So
a=100,
b=100.

Two trains are running in the opposite direction, So (u+v).
u+v=x+2x
=3x.

Time = 8sec
a+b/u+v = time
100+100/3x = 8;
200/3x = 8;
X = 25/3 m/sec;
X = 25/3*18/5 km/hr,
= 30 km/hr.

Speed of faster train = 2x.
=2*30.
=60km/hr.

@Nutan.

How come 18/3?

Answer will be 30km/hr.
Because the simplification of 200/24 = 25/6.
But in the answer it's given as 25/3 that's why the answer is changed to 60 km/hr.

Two train length is 100 m long each.
So, 100 + 100 = 200m.
Cross eachother in 9 sec so to find speed.

Speed = length/time.
Speed here ( one is moving twice as fast as the other).

If one train is x then another should be twice of x so 2x.

They are moving in opposite directions so x + 2x = 3x ( when moving in same direction we substract { u - v} in case of opposite direction { u+v}.

3x = 200m/9 sec * 18/5 (But the answer should be in km/s so we use 18/5).
3x = 90km/hr,
x = 90/3,
x = 60km/hr.

How come 50/3? Please explain me.

If x= 25/3, how is 2x= 50/3?

It should have been 50/6. Am I right?

Can someone please tell me, we are already multiplying 2 with x then why we are again multiplying with 2 with 25/6? Please explain me.

@Fabarah Israr.

It's not 25/6 it's 25/3.

And it is asked to calculate the speed of a faster train.
x= 25/3 (speed of the slower train)
whereas the speed of the faster train is 2x, therefore 2*25/3 gives 50/3.