QR = PS = q
QM = (1/5)*QR = q/5;
PM = PQ + QM = p + q/5;
PM = PS + SM; SM = PM - PS = p + q/5 - q = p - (4/5)*q
QR = PS = q
QM = (1/5)*QR = q/5;
PM = PQ + QM = p + q/5;
PM = PS + SM; SM = PM - PS = p + q/5 - q = p - (4/5)*q