An electromagnet, which is battery operated, attracts a ball (monkey). After firing the projectile, the electromagnet circuit breaks.
The monkey falls and accelerates downwards at g. The bullet starts off travelling along the aiming line, but it is also accelerated downwards at g. So its motion as 'falling below the aiming line'. At equal times, it will fall below this line by an amount equal to the distance fallen by the monkey. At the time when both have the same horizontal position, they will both have fallen the same distance. The monkey and the bullet collide.
PHYSICS, Class XI, Part-2,Chapter-4, Motion in a plane.