Postal or Sorting Assistants Model Paper 2 With Solutions

(b) Part of the tank filled by the three pipes working
simultaneously in one hour is=$\frac{1}{5}+\frac{1}{6}-\frac{1}{15}=\frac{3}{10}$
i.e. it takes $\frac{10}{3}$ hours to fill up the tank completely
Now, $\frac{1}{2}$ of the tank is filled with all the pipes open,
simultaneously together in $\frac{10}{3}\times \frac{1}{2}=1\frac{2}{3}$hours