A rule of three is used to calculate ratios proportional to each other. Proportional means that when a quantity A is doubled, B is also doubled. For example, e.g. A train with constant speed drives in an hour 80km so it drives in two hours 160km.
Example: A train runs in 2 hours 140 km then 2 has to be set for A and 140 for B. The route is asked for 3 hours. So 3 for C and x is the distance searched.
So is x= 210 that means the train runs 210km in 3 hours.
An inverse rule of three is a calculation scheme for the calculation of ratios which are indirectly proportional to each other. Indirectly proportional, when a quantity A is halved, the quantity B is doubled. If 2 trucks need to transport a certain amount of overburden 10 days so 1 truck needs double time so 20 days.
Example: If you need 4 trucks to remove a certain amount of overburden 10 days then 2 is set for A and 10 is for B. Looking for the time that 2 trucks is needed so C is set to 2.
So is x= 20 that means a truck needs 20 days.