Two taps can fill a cistern in 30 and 40 minutes respectively. If both the taps are opened simultaneously then the approximate time taken to fill the cistern is -

We know that,
Two pipes A and B can fill (or empty) a tank in X and Y minutes respectively, while working alone.
If both the pipes are opened together,
then the time taken to fill (or empty) the cistern is given by XY/(X+Y) minutes.
Here,
X = 30 minutes and Y = 40 minutes
Therefore,
the required time = (30 x 40)/(30 + 40)
= 1200/70
= 120/7
= 17(1/7) minutes.
Hence the answer is 17(1/7) minutes.