Pipe A can fill a tank 6 times faster than a pipe B. If B can fill a tank in 21 minutes, then  time taken by both the pipes to fill the tank is :

3 minutes

The correct answer is Option (2) → 3 minutes

When both pipes work together, the combined rate is,

$\frac{2}{7}+\frac{1}{21}=\frac{7}{21}=\frac{1}{3}$ per minutes

The time taken to fill the tank = $\frac{1}{\frac{1}{3}}=3$ minutes