the atmospheric pressure is 75cm of Hg which means that on inverting the 50cm tube into the mercury trough, the mercury should ideally rise upto a height of 75cm above the mercury level in the trough. since the tube is only 50cm long and half of it i submerged, the mercury will rise upto 25cm above the level in the trough i.e. the whole of the tube would get filled with mercury.