The time they take to complete the job together is $2\text{ hours}$, which is exactly half of the harmonic mean of their individual completion times.

The combined work rate of A and B is the sum of their individual work rates, which equals $\frac{1}{2}\text{ job per hour}$.

The time they take to complete the job together is the arithmetic mean of their individual completion times, which equals $4.5\text{ hours}$.

The time they take to complete the job together is given by the expression $\left(\frac{1}{t_A} + \frac{1}{t_B}\right)^{-1}$, where $t_A$ and $t_B$ are their individual completion times.