s MRS= -P1/P2 or is MRS= -Mux/MUy
The correct formula for the marginal rate of substitution (MRS) is:
MRS = -MUx/MUy
Where MUx is the marginal utility of good x and MUy is the marginal utility of good y.
MRS = -MUx/MUy
Where MUx is the marginal utility of good x and MUy is the marginal utility of good y.