Two cars crash into each other at an intersection on a very icy road. One 3,000 lb car is traveling 45 mph NW. The other, weighing 2,000 lb, is traveling 30 mph E. After the collision, their bumpers lock together. What is the velocity of the wreck as it slides off the road ?

Answer 1

The final velocity equals the total momentum (which stays the same) divided by the combined mass (5000 lb/g or 155.3 slugs)

Add, as vectors, the two initial momenta of the two cars to get the total momentum.

In this case you can keep the speeds in mph and the masses in lbm (pounds-mass), to get a final result in mph. In general it is safest, when working in British-American units, to convert speed to ft/s and mass to slugs.