[填空题]
A neon atom (m=20u) makes a2iz k a*kl3vc-l;5to wkqi(2h perfectly elkw;+iz+ :hnnjtti(xbbz 71 jm. 8w/y sastic collision with another atom at rest. After the impact, the neon atom travels away ixs. t( 8 t;+zhj+zmnwyn /bb:kw17ijat 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
