A tank can be filled by two taps X and Y in 5 hrs and 10 hrs respectively while another tap Z empties the tank in 20 hrs. In how many hours can the tank be filled if all 3 taps are kept open?
1 hour work of X, Y and Z =Â $$\frac{1}{5}+\frac{1}{10}-\frac{1}{20}$$ =Â $$\frac{5}{20}=\frac{1}{4}$$
So, they will take 4 hours to fill the tank.
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