Ice cream sells for Rs. 20. Mohini who likes ice- cream has already consumed 4. Her Marginal utlility of one rupee is 4. Should she consumes more ice cream or stop consuming and Why?

A consumer strikes his equilibrium when,  $M{U}_M=\frac{M{U}_X}{P_X}$
$$\begin{gathered} 4=\frac{M{U}_X}{20} \\ M{U}_X=80 \end{gathered}$$

It implies that at equilibrium, marginal utility derived from ice cream must be equal to 80.
So we can say that, if 4 units of ice cream yield satisfaction of 80 utils then she must stop consuming more. But if ​marginal utility derived from ice cream is greater than 80 then she should consume more ice creams to strike equilibrium.