What is the car's speed

A 2.00 x 10^4 kg railroad car is rolling at 1.00 m/s when a 5000 kg load of gravel is suddenly dropped in. What is the car's speed just after the gravel is loaded?

Thanks :)

(20,000 x 1)/(20,000 + 5,000) = 0.8m/sec.

Yeah this is right answer (20,000 x 1)/(20,000 + 5,000) = 0.8m/sec.