In the equation y = 10x, the constant of proportionality is the number 10.
This equation describes a linear relationship between the variables y and x. The constant of proportionality indicates how much y changes for a unit change in x. In this case, for every increase of 1 in x, y increases by 10.
Therefore, the number 10 acts as a scaling factor that defines the rate at which y is proportional to x.