1) There is a relationship between speed, distance and time:
Speed = Distance / Time OR
Distance = Speed* Time
2) Average Speed = 2xy / x+y
where x km/hr is a speed for certain distance and y km/hr is a speed at for same distance covered.
**** Remember that average speed is not just an average of two speeds i.e. x+y/2. It is equal to 2xy / x+y
3) Always remember that during solving questions units must be same. Units can be km/hr, m/sec etc.
**** Conversion of km/ hr to m/ sec and m/ sec to km/ hr
x km/ hr = (x* 5/18) m/sec i.e. u just need to multiply 5/18
Similarly, x m/sec = (x*18/5) km/sec
4) As we know, Speed = Distance/ Time.
Now, if in questions Distance is constant then speed will be inversely proportional to time i.e. if speed increases ,time taken will decrease and vice versa.