Two values are proportional to each other if the division of a value of the first group by the corresponding number of the second group always gives the same number. This number is then called factor of proportionality.
Proportionalities
What is a proportionality?
A proportionality is a relation fulfilling: The more one value grows, the more another value grows. E.g. if you buy more fruits, the price of the fruits will also increase.
How to calculate with proportionalities?
First find out what the proportionality factor is. This the value gotten by dividing two corresponding values.
It is easy to find values corresponding to other numbers: Just multiply or divide by the factor of proportionality.