[填空题]
A neon atom (m=20u) makes a perfectly gk)2 hkw:y8 zkelastic collision witw.*n)7 k c.f fswrb1chh another atom at rest. After the impact, the neon atom travels away ) fnch w .71rsbw*f.kcat 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
