[填空题]
A neon atom (m=20u) m iodquo:a1p 48fu6 k,aakes a perfectly elastic collision with g;nk,)n,e6ctobq m2sjsb +(xanother atom at rest. After the impact, the neon atom travels away at gex)nkco2+j b(bms ;6n,tq ,s 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
