(d) 30 minutes
Let the total time taken to fill the tank be x minutes.Then,
work by B in half hour + work done by (A + B) in half hour = 1
\(\big(\frac{1}{40}\times\frac{X}{2}\big)\) + \(\big(\frac{1}{60}+\frac{1}{40}\big)\) x \(\frac{X}{2}\) = 1
\(\Rightarrow\) \(\frac{X}{80}\) + \(\frac{X}{48}\) = 1 \(\Rightarrow\) \(\frac{3X+5X}{240}\) = 1
\(\Rightarrow\) 8x = 240 \(\Rightarrow\) x = 30 min.