To solve this problem it is necessary to apply the conservation equations of the Momentum. The equation that represents such momentum conservation is given by,
Final momentum = Initial momentum

Where,
=mass light
= mass heavy
= velocity of mass light
= velocity of mass heavy
= final velocity
One of the masses is heavier than the other previously detached so we will assume that mass is 1/3 of the total, and the heavy mass is 2/3 of the total.
We start finding the initial velocity, then


The distance traveled is then calculated by the kinematic equations of motion,



The speed of the lighter piece is given also for the kinematic equation of movement,




Replacing at the conservation of momentum equation we have,




Therefore the speed of the heavier fragment just after the explosion is 18.52m/s against the direction of movement.