A bullet of 60 gm is fired using a rifle of mass 12 kg. Rifle recoils with a velocity of 2.5 m. Per sec. calculate initial velocity of the bullet

Like your subject title says - use the principle of conservation of momentum to figure it out. The system starts with zero momentum. so after the bullet is fired the sum of the bullet's momentum plus the gun's momentum = 0. Do you know how to calculate gun's momentum?