Pipes and Cisterns - Time to Fill Tank

Tap A fills $\frac{1}{10}$ of tank/min, Tap B fills $\frac{1}{30}$ of tank/min. In 5 minutes together, fraction filled = $5 \times \left(\frac{1}{10} + \frac{1}{30}\right) = 5 \times \frac{4}{30} = \frac{20}{30} = \frac{2}{3}$. Remaining fraction = $1 - \frac{2}{3} = \frac{1}{3}$. Time required by Tap B alone to fill remaining fraction = $\frac{1/3}{1/30} = 10$ minutes.