Tap A fills the tank in 11 hours, Tap B empties the tank in 22 hours.

Use the formula $\frac{\text{Product}}{\text{Difference}}$ when one tap fills and another empties: $\frac{11 \times 22}{22 - 11} = \frac{11 \times 22}{11} = 22$ hours.

Students often add the rates ($\frac{1}{11} + \frac{1}{22}$) thinking both are filling, forgetting that the outlet tap subtracts from the total efficiency.

This concept of net efficiency is identical to problems involving relative speed where two objects move in opposite directions on a track.