Speed, time and distance are linked by Distance = Speed × Time. Most problems are solved by setting up this relation for two situations and comparing them.
1Basic relations
Speed = Distance ÷ Time, Time = Distance ÷ Speed, Distance = Speed × Time
x km/hr = x × 5/18 m/s; x m/s = x × 18/5 km/hr
2Average speed
Average speed = total distance ÷ total time
Equal distances at x and y km/hr: average speed = 2xy/(x + y)
Example
Going at 40 and returning at 60 gives 2·40·60/100 = 48 km/hr, not 50.
3Ratios
If the speed ratio is a : b, the time ratio for the same distance is b : a
Moving at a/b of the usual speed makes the time b/a of the usual time