A vertical tube is closed at one end and open to air at the other end. The air pressure is 1.01 × 105 Pa. The tube has a length of 0.75 m. Mercury (mass density = 13 600 kg/m3) is poured into it to shorten the effective length for standing waves. What is the absolute pressure at the bottom of the mercury column, when the fundamental frequency of the shortened, air-filled tube is equal to the third harmonic of the original tube?