Let the pipes fill the tank in x minutes. Part of tank filled by pipes A and B in one minute: $$\frac{1}{{36}}$$Part of the tank filled by pipe A in 1 minute = $$\frac{1}{{36}} - \frac{1}{x}$$According to the question,$$\eqalign{ & 30 \times \frac{1}{{\text{x}}} + 40 \left( {\frac{1}{{36}} - \frac{1}{{\text{x}}}} \right) = 1 \cr& \Rightarrow \frac{{30}}{{\text{x}}} + \frac{{40}}{36} - \frac{{40}}{{\text{x}}} = 1 \cr & \Rightarrow \frac{{30}}{{\text{x}}} + \frac{{10}}{9} - \frac{{40}}{{\text{x}}} = 1 \cr & \Rightarrow \frac{{10}}{{\text{x}}} = \frac{1}{9} \cr} $$Hence, x = 90 minutes.Alternatively,% of tank filled by (A+B) in 1 minute = $$\frac{{100}}{{36}}$$ = 2.78%% of tank filled in 30 minutes = 2.78 × 30 = 83%That means 17% of tank can be filled by A in last 10 minutesThen, % tank filled by A in 1 minute = $$\frac{{17}}{{10}}$$ = 1.7%% of tank filled by B in 1 minute = 2.78 - 1.7 = 1.1%So, B alone can fill 100% tank in,= $$\frac{{100}}{{1.1}}$$ = 90 minutes (approx.)