A 485 kg boxcar traveling at 23 m/s strikes a second car (initially at rest). The cars couple, and move off at 9 m/s. What is the mass of the second car?

Since momentum is conserved during the brief collision,

485*23 = (485 + M)*9

Solve for M, the mass of the second car.

485 + M = 1239.4
M = 754.4 kg

Since there is only one significant figure in the final speed, it would be better to say M = 750 kg