Pipes A and B can fill a tank in 20 hours and 30 hours respectively and pipe C can empty the full tank in 40 hours. If all the pipes are opened together, how much time will he needed to make the tank full ?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → $17\frac{1}{7}$ hours
Filling Rate of A = $\frac{1}{20}$ tank per hour.
Filling Rate of B = $\frac{1}{30}$ tank per hour.
Filling Rate of C = $\frac{1}{40}$ tank per hour.
Net Rate = Rate of A + Rate of B - Rate of C
$=\frac{1}{20}+\frac{1}{30}-\frac{1}{40}$
$=\frac{7}{20}$
$\text{Time} = \frac{1}{\text{Net Rate}}=\frac{7}{20}=17\frac{1}{7}$ hours