[填空题]
A neon atom (m=20u) makes a perfectly elcijq0g*5x3*ps mu(mn nr65oc1 e v;nf7v(ym j **xzfsastic collision with another atom at rest. After the impact, the neov7*z mfs(*xy jn 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
