The number of days the water supply lasts is in direct proportion with the liters of water the elephant drinks.
In fact, suppose that there are [tex] x [/tex] liters of water remaining, and the elephant drinks [tex] y [/tex] liters of water each day.
This means that the elephant will finish the water in
[tex] d = \dfrac{x}{y} \iff x = dy[/tex]
So, the more liters are there, the more days it will take to the elephant to finish the water. In general, every time the ratio of two quantities is constant, they are in direct proportion.
