1) Two pipes can fill a tank in $10$ hours and $12$ hours respectively while a third pipe empties the full tank in $20$ hours. If all the three pipes operate simultaneously, in how much time will the tank be filled ? A) 7 hrs 30 min B) 7 hrs 40 min C) 7 hrs 50 min D) 8 hrs 30 min E) 7 hrs Answer: Option AExplanation:Net part filled in $1$ hour $=\left(\frac{1}{10}+\frac{1}{12}-\frac{1}{20}\right)$ $=\frac{8}{60}$ $=\frac{2}{15}$ The tank will be full in $\frac{15}{2}$ hrs = 7 hrs 30 min
