The local animal shelter throws a dog-themed party. Humans, h, and dogs, d are both invited. The event space imposes two restrictions on the party: there can only be 120 dogs and humans combined, and, to keep things manageable, there must be 1 human to every 3 dogs.

This situation can be represented by a system of two linear equations. One of these equations is given. Write the second equation in the box.
d/3=h