RRB Paramedical Staff Nurse Model Paper 3

According to the question.
Part of the cistern filled by taps A,B and $\mathit{C}$ in 1 min $=\frac{1}{10}$
Part of the cistem filled by taps B and C in 1 min
$=\frac{1}{30}+\frac{1}{40}$
$=\frac{4+3}{120}=\frac{7}{120}$
Thus, part of the cistem filled by tap C in 1 min
$\left(\mathit{A}+\mathit{B}+\mathit{C}\right)-\left(\mathit{A}+\mathit{B}\right)$
$=\frac{1}{10}-\frac{7}{120}=\frac{12-7}{120}$
$=\frac{5}{120}=\frac{1}{24}$
Hence tap C will fill the cistem in 24 min.