Problem on Train Important Formula for Interview Aptitude question Formula

For converting Km/Hour to Meter/Second
x Km/hr=(x*5/18)m/s
For converting meter/second to Km/Hour
y m/s=(y*18/5)Km/hr
How to remember this formula
First converting kilometer to meter
it is way of converting High to low
So multiply by the low number 5/18
Second converting meter to Kilometer
It is way of converting low to high
So multiply by the high number 18/5
Time taken by train of length l meters to pass a stationary object of length b meters I s the time taken by the train to cover (l+b) meters
Suppose two train are moving in same direction with U m/s and V m/s where u>v then their relative speed is (u-v)m/s (u>v)
If the train going in opposite direction then there relative speed is =(u+v) m/s
Two train of length a meters and b meters are moving in opposite direction at um/s and v m/s then time taken to cross each other =(a+b)/(u+v)
If it is going same direction=(a+b)/(u-v) sec