Direct proportionality - math word problems - page 18 of 35

Two sequences of numbers are proportional if their corresponding elements have a constant ratio. Direct proportionality is the dependence of two quantities, such that the number of times one quantity increases, the other quantity increases as many times. In other words: direct proportionality is a relationship in which it applies: in what proportion one quantity changes, in that proportion the other quantity also changes.

For example:
For 1 euro, I buy 10 rolls, then for 2 euros I buy 20 rolls in the same store.
A car travels at a constant speed, then the distance traveled is directly proportional to the time spent traveling, with the speed being the constant of proportionality.