assuming a completely elastic collision (like steel ball striking steel ball, not steel ball striking marshmallow fluff) I think we're dealing with conservation of momentum.
p=mv, and in a closed system, it's a constant. unlike kinetic energy.
v_{1,f} = \left( \frac{m_1 - m_2}{m_1 + m_2} \right) v_{1,i} + \left( \frac{2 m_2}{m_1 + m_2} \right) v_{2,i} \,
v_{2,f} = \left( \frac{2 m_1}{m_1 + m_2} \right) v_{1,i} + \left( \frac{m_2 - m_1}{m_1 + m_2} \right) v_{2,i} \,
I don't think that's going to read very well, so check out here:
http://en.wikipedia.org/wiki/Momentum
What's going to be the interest in the next hour?
JP