Find average speed from distance and time.
Three quantities, one relationship, three rearrangements.
Speed = distance ÷ time · Distance = speed × time · Time = distance ÷ speed
The frequent error is mixing units. If distance is in miles and time in minutes, the answer is miles per minute, not miles per hour. Convert minutes to hours by dividing by 60 before calculating, or multiply the result by 60 afterwards.
Drive 60 km at 60 km/h and then 60 km at 120 km/h. The average speed for the journey is not 90 km/h.
The first leg takes one hour, the second half an hour. Total distance 120 km, total time 1.5 hours, average speed 80 km/h. Because you spend less time at the higher speed, it contributes less to the average.
Average speed = total distance ÷ total time, always.
The practical consequence is that speeding saves less time than people expect. Raising the average from 100 to 120 km/h over 50 km saves five minutes — and that assumes no traffic, no lights, and no enforcement. On journeys with any interruption, the saving usually disappears entirely.
Kinetic energy rises with the square of speed, so stopping distance grows far faster than speed does. Doubling speed roughly quadruples the braking distance, before adding thinking distance, which scales linearly.
At 50 km/h a typical total stopping distance is around 25 metres; at 100 km/h it is closer to 80 metres. This is why small increases in speed produce disproportionate increases in collision severity, and why urban speed limits are set where they are.