Toss Up - Physics - Multiple Choice

A car of mass 700 kilograms travels at 20 meters per second and collides with a stationary truck of mass 1400 kilograms. The two vehicles interlock as a result of the collision and slide along an icy road. Which of the following is the approximate velocity in meters per second of the combined system?
W) 5.6
X) 6.6
Y) 7.6
Z) 8.6