Let the first car's average speed be x
The second car's speed would be x+5




So the speed of the slower car is 50mph, and the speed of the faster car is 55mph
if the velocity of the car reduces from 70km/h to 50km/h then the speed of the car will be equal to the speed of the lorry...
thus the relative velocity will be 0
To measure the fluids that you will be using such as water or other chemicals if your mixing them and they have to be precise measurements