[填空题]
A neon atom (m=20u) mht/h5dnoitvb -zo*2z q( cj**a6 tb;r;xzy.b 96*mu l4maq1 lbiux+izkes a perfectly elastic collision with another atom at rest. After the impact, the neoq6ut61;mx +rm9yb*4 iau. z lblzib ;xn 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
