Ball A, with a mass of 10kg, is moving to the right at 20 m/s. Ball B is moving to the left at 20 m/s. Upon collision, Ball B comes to a standstill, and Ball A moves to the left at twice its original speed. What is the mass of Ball B?

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While we're happy to help, we wont do all the work for you.

I truly don't know. The answer I originally came up with was wrong but it doesn't tell me why. I want to better understand why I got it wrong.

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I just realized what I got wrong! Sorry for the trouble.