[填空题]
A neon atom (m=20u) makes asxc t0 xa7 yl3/mmtq:7ju1eq3 7 js4um72 esnrhk15j7x+hv zw perfectly elastic collision with another atom at rest. After the impact, the neon asx r7zhvjw+7 15k 2nehtom 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
