A gun mounted on the top of a moving truck is aimed in the backward direction at an angle of 30 degrees to the vertical.If the muscle velocity of gun is 4m/s, the value of speed of truck that will make the bullet come out vertically is

firstly, you should concentrate on terms '' muzzle velocity'' and ''aimed in''.Muzzle velocity means velocity WITH RESPECT TO the gun and aimed in means angle made by bullet as seen from truck...

to find velocity of truck that will make bullet come out vertically(as seen from ground frame ), velocity of truck must be equal to horizontal component of velocity of bullet wrt truck i.e 4cos(90-30)=2 m/s......