$\displaystyle A$ and $\displaystyle B$ can paint a room in $\displaystyle \frac{12}{7}$ hours.

$\displaystyle B$ and $\displaystyle C$ can paint the room in $\displaystyle 6$ hours.

$\displaystyle A$ and $\displaystyle C$ can paint the room in $\displaystyle 12$ hours.

How long would each take to paint the room if working alone?

[Any comments on $\displaystyle C$ ?]