[填空题]
A neon atom (m=20u) makes a perfec 3vo*5 -hv9-4lrdjxw)s mhm nbtly elastic collzj- t,fpba;4x** , 3 e ubx0+wvtodzneision with another atom at rest. After the impact, the neo0f;*,jonx t*u + ,z3-xep advwteb4bzn 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
