What strategies in finding equations to express relationships?

The strategies that will help in finding equations to express relationships is that you need to have some variables.

For example, x and y. A relationship can either be direct proportional or inversely proportional. When we say that it is directly proportional, as the value of x increases, the value of y also increase. Inversely is the opposite one.

These are the multiples of 15 but what you are probably asking or what are the factors of 15