Final answer:
The constant of proportionality in the equation Y = -3/10x is -3/10, representing the rate at which Y changes with respect to x in a directly proportional relationship.
Step-by-step explanation:
The constant of proportionality in the equation Y = -3/10x is -3/10. This value represents the constant rate at which Y changes in relation to x. In direct proportionality, expressed as y = kx, the constant k is the constant of proportionality. Here, -3/10 is the value that defines the ratio of the variation of Y in comparison to x. If you plot this relationship on a graph with Y on the Y-axis and x on the X-axis, the line would slope downward, since the proportionality constant is negative, and cross the origin (0,0) because there is no added constant on either side of the equation.