[填空题]
A neon atom (m=20u) makes a perfectly elmdy2ik1e2z by0f;zfu7sb ;g7astic collinha4y)7y) ptm sion with another atom at rest. Af)yh 7mt)4y pnater the impact, the neon atom travels away at a 55.6 $^{\circ} $ angle from its original direction and the unknown atom travels away at a -50 $^{\circ} $ angle. What is the mass (in u) of the unknown atom? [Hint: You can use the law of sines.](Let A represent the incoming neon atom, and B represent the target atom) $m_B$ u
