RRB Paramedical Staff Nurse Model Paper 4

According to the question
Part filled by A from 8 am to 11 am
$=\frac{3}{15}=\frac{1}{5}$
Part filled by B from 9 am to 11 am
$=\frac{2}{12}=\frac{1}{6}$
Part filled till 11 am
$=\frac{1}{5}+\frac{1}{6}$
$=\frac{6+5}{30}=\frac{11}{30}$
At 11 am pipe $\mathit{C}$ is opened to empty it.
Thus part of tank emptied in 1h
$=\frac{1}{12}+\frac{1}{15}-\frac{1}{4}$
$=\frac{-6}{60}=\frac{-1}{10}$
Since, negative sign shows that full tank will be emptied in 10h
Thus $\frac{11}{30}$ part will be emptied in
$=\frac{11}{30}\times 10$
$=\frac{14}{3}\mathit{h}$
le. in 3 h 40 min, i.e. at 11:40 am