Helen's preferences over CDs (C) and sandwiches (S) are given by U(S, C) = SC + 10(S

Question:

Helen's preferences over CDs (C) and sandwiches (S) are given by U(S, C) = SC + 10(S + C), with MUC = S + 10 and MUS = C + 10. If the price of a CD is $9 and the price of a sandwich is $3, and Helen can spend a combined total of $30 each day on these goods, find Helen's optimal consumption basket.