Aptitude - Problems on Trains - Discussion

See the answer for 18/5 is how?
1 km=1000m & 1hr=3600sec by this we get 1km/1hr = 5/18 m/sec.But
the answer must be in km/hr....But 5/18 is in m/sec. we have to convert it to km/hr.
1 km = 1000m => m = 1 km/1000 hr/3600)
= (1km/1000)*(3600/1hr)
= 3600km/1000hr => 18/5 km/hr
friends...its a perfect answer...have a look.

I cant understand relative speed formula ie) x-5 is this standard formula ?

(x-5) is not a standard formula x is taken as speed of train and 5 is man's speed, to calculate train's speed from relative speed we subtract (x-5).

x - 5 = 45
x= 45+5
x = 50 km/hr.

relative speed when two movable things whose speeds x,y are moving in same direction is =X-Y,
relative speed when two movable things whose speeds x,y are moving in OPPOSITE direction is =X+Y
Its an standard formula

5/18 is conversion of km/hr to m/s and 18/5 is conversion digit m/s to km/s

DONT WORRY FRIENDS ITS VERY SIMPLE

instead of going for a new formula.we can simply get the answer by using the standard formula.i,e

t=(a+b)/(u-v)
u can see this formula from the general formulas of this exercise.
where
t-time taken by the train to cross the man=10 sec;

a-length of the train=125meters;
b=0(because the train is supposed to cross a person);
u=speed of the train(you are supposed to find out);
v=speed of the man who is running=5km/hr=5*(5/18)=25/18m/sec;

by substituting the values in the above equation you can get the value..

Nice explanation friends. !

Both (train and man) moves in same direction so we subtract.

Both moves in opposite direction means we add it.

We convert m into km, sec into hour so we multiply 18/5.

Thanks to all.

I got many idea's to solve this problem.