A railroad hopper car filled with sand is rolling with an initial speed of 15 m/

Question

A railroad hopper car filled with sand is rolling with an initial speed of 15 m/s on straight, horizontal tracks. You can ignore frictional forces on the railroad car. The total mass of the car plus sand is 85,000 kg. The hopper door is not fully closed so sand leaks out the bottoms. After 10 minutes, 13,000 kg of sand has leaked out.

So I thought that because it does not say that it is elastic or inelastic that I should not assume that KE is conserved or anything is conserved except for mass and momentum. So I tried to do:
Mass of hopper and sand * initial v = (mass of hopper and sand -sand that has leaked)* final v.

that gave me that the speed of the hopper was 17.7m/s. And that was wrong.
Any ideas how to approach the problem?

Explanation / Answer

The mass of car+sand is M = 85000 kg The speed of car+sand is u = 15 m/s As there are no forces acting on the car, there will be no change in its velocity according to Newton's first law of motion. The car has velocity 15 m/s after the sand was lost

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