Challenge Level

$-40$

**Why?**

$\begin{align}0^\circ C&= 32^\circ F\\

+100\downarrow\hspace{6mm}&\hspace{8mm}\downarrow+180\\

100^\circ C &= 212^\circ F\end{align}$

$+100^\circ C$ is the same as $180^\circ F$

$+10^\circ C$ is the same as $+18^\circ F$

$-10^\circ C$ is the same as $-18^\circ F$

$-10^\circ C=32-18^\circ F = 14^\circ F$

$-20^\circ C=14-18^\circ F = -4^\circ F$

$-30^\circ C=-4-18^\circ F = -22^\circ F$

$-40^\circ C=-22-18^\circ F = -40^\circ F$