AMC 8 1990 Problem 10

On this monthly calendar, the date behind one of the letters is added to the date behind $\text{C}$. If this sum equals the sum of the dates behind $\text{A}$ and $\text{B}$, then the letter is

$\text{(A)}\ \text{P} \text{(B)}\ \text{Q} \text{(C)}\ \text{R} \text{(D)}\ \text{S} \text{(E)}\ \text{T}$