Assume the demand function for good X can be written as Qd

Assume the demand function for good X can be written as Qd = 80 - 3Px + 2Py + 10I, where Px = the price of X, Py = the price of good Y, and I = Consumer income. According to this equation: a rise in the price of Y would cause the demand for X to decrease. X and Y are complements X is an inferior good. X and Y are substitutes.

Solution

Rise in price of x would increase demand for x and rise in price of y would increase demand for x

Hence x and y are substitutes