Question

A firework rocket is launched vertically from rest and loses 60% of its mass during a burn time of 4 seconds. What is the final speed of the rocket relative to the ground (in m/s) when the gas relative to the rocket has a speed v= 80 m/s?
a. 15 m/s; b. 34 m/s; c. 68 m/s; d. 180 m/s
Thanks for your help.