RRB Junior Engineer Stage 1 Model Papers 8

In one minute (A + B) can together fill $\frac{1}{36}+\frac{1}{45}=\frac{1}{20}$ part.
In 7 minutes part of tank filled $=\frac{7}{20}$
Remaining part $=1-\frac{7}{20}=\frac{13}{20}$
In 8th minutes, part filled by A, B and C altogether $=\frac{1}{36}+\frac{1}{45}-\frac{1}{30}=\frac{1}{20}-\frac{1}{30}=\frac{1}{60}$
$\frac{13}{20}$ part of tank filled by $\left(\mathit{A}+\mathit{B}+\mathit{C}\right)$ $=60\times \frac{13}{20}=39$minutes