Final answer:
The equation with a constant of proportionality equal to 5 is y = 5x.
Step-by-step explanation:
The equation that has a constant of proportionality equal to 5 is y = 5x. In a direct proportion, when one variable (x) changes, the other variable (y) changes in the same ratio. The constant of proportionality (k) represents this ratio, and in this case, it is equal to 5.