A bomb is projected vertically up with a velocity of $100 m/s.$ After $8\; s$ it blasted into two pieces of mass in the ratio $1:2$ . The lighter piece goes it with the velocity $40 \;m/s$ then what is the velocity of the heavier piece just after blasting?

Question

$$\begin {array} {1 1} (a)\;10\;m/s\;up & \quad (b)\;10\;m/s\;down \\ (c)\;20\;m/s & \quad  (d)\;20 m/s\;down \end {array}$$