Answer:
50
Step-by-step explanation:
In maximizing utilty the ratio of marginal utilities of both goods, which is called the Marginal Rate of Substitution (MRS), must be equal to the prices ratio.
Here, the ratio of prices (cupcakes over cheese) is 3/5. So, the MRS (marginal utility of cupcakes (Ucc) over marginal utility of cheese(Uch))) must be 3/5.
As Ucc=30, then 30/Uch = 3/5. So, Uch = 50.