Postal or Sorting Assistants Model Paper 3 With Solutions

(d) Part of the tank filled by the three pipes working
simultaneously in one hour is $=\frac{1}{5}+\frac{1}{6}-\frac{1}{12}=\frac{17}{60}$
i.e. it takes $\frac{60}{17}$ hours to fill up the tank completely.
Now, $\frac{1}{2}$ of the tank is filled with all the pipes open,
simultaneously together in $\frac{60}{17}\times \frac{1}{2}=1\frac{13}{17}$ hours