Two pipes can fill a tank in 20 minutes and 30 minutes respectively. If both the pipes are opened simultaneously, then the tank will be filled in            

$Afillthe\text{\hspace{0.17em}tank}=20\min \Rightarrow 1\text{\hspace{0.17em}\hspace{0.17em}minute}=\frac{1}{20}$

$Bfillthe\text{\hspace{0.17em}\hspace{0.17em}tank}=30\min \Rightarrow 1\text{\hspace{0.17em}\hspace{0.17em}minute}=\frac{1}{30}$

$VolumefilledbybothA\&B\Rightarrow A+B=\frac{1}{20}+\frac{1}{30}=\frac{3+2}{60}=\frac{5}{60}$

$\therefore Totaltime=\frac{1}{\frac{5}{60}}\Rightarrow \frac{60}{5}=12\text{\hspace{0.17em}\hspace{0.17em}minutes}$