A neon atom mm$=20\mathrm{.}0$ uu makes a perfectly elastic collision with another atom at rest. After the impact, the neon atom travels away at a $55\mathrm{.}6$ angle from its original direction and the unknown atom travels away at a $-50\mathrm{.}0$ angle.What is the mass $($in uu$)$ of the unknown atom? $[$Hint: You could use the law of sines.$]$

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mm $=$

$21\mathrm{.}4$

uu