Let the capacity of tank be 180 litres.
1st tap fills 180 litres in 15 minutes, 1min= 12 ltres fill
2nd tap fills 180 litres in 12 minutes, 1min= 15 ltres fill
3rd tap empties 180 litres in x minutes [let]... 1 min empty = \(\frac{180}{x}\) litres
Total filling time = 20 minutes
\(\Rightarrow\;\)\(20\times12 + 20\times15 - 20\times\frac{180}{x}\) = \(180\)
\(x = 10\) minutes
The correct option is (B): 10 minutes