Solution:
Since there exists a positive relationship between price of ale and demand for cakes, the two are substitutes.
After substituting the given values,
Qc = 100 β 30pc + 20 pa
Where, qc= quantity of the cake, m = income of percy, pa = price of ale, pc = price of cake
Qc = 120 β 30 pc
Now, Rearranging demand function calculated above,
Qc = 120 β 30 pc
30 pc = 120 β Qc
Thus, Pc = 4 β 0.033 Qc