Consider the following LPP: Maximise \( Z = 9x + 3y \) Subject to the constraints: \[ x + 3y \leq 60, \quad x - y \leq 0, \quad x \geq 0, \quad y \geq 0 \] If \( x = A, y = B \) is the optimum solution of the given LPP, then the value of \( A + B \) is: