
$$\begin{aligned}
1+2 & =3 \\
4+5+6 & =7+8 \\
9+10+11+12 & =13+14+15\\
\vdots
\end{aligned}$$ 

If this pattern is continued, find the last number in the $80$th row (e.g. the last number of the third row is $15$).

Let's think step by step and output the final answer within \boxed{}.
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