RRB Paramedical Staff Nurse Model Paper 4

According to the question,
Part of the tank filled by the pipe in $1\mathit{h}=\frac{1}{3}$
Because of leak, part of the tank filled by the pipe in $1\mathit{h}=\frac{2}{7}$
Thus, part of the tank emptied by the pipe in $1\mathit{h}$
$=\frac{1}{3}-\frac{2}{7}$
$=\frac{7-6}{21}=\frac{1}{21}$
Hence, the leak will empty the tank in $21\mathit{h}$