# Pipes and Cisterns Questions Answers

Pipes and Cisterns Questions Answers (MCQ) listings with explanations includes questions of calculating filling of tanks, getting tank empty by pipes etc which are important for many competitive exams.

11) A tap can fill a tank in 6 hours. After half the tank is filled then 3 more similar taps are opened. What will be total time taken to fill the tank completely

• 1) 2 hours 30 mins
• 2) 2 hours 45 mins
• 3) 3 hours 30 mins
• 4) 3 hours 45 mins
Ans.   D
Explanation :
Half tank will be filled in 3 hours Lets calculate remaining half, Part filled by the four taps in 1 hour = 4*(1/6) = 2/3 Remaining part after 1/2 filled = 1-1/2 = 1/2 \begin{aligned} \frac{2}{3}:\frac{1}{2}::1:X \ => X = \left( \frac{1}{2}*1*{3}{2} \right) \=> X = \frac{3}{4} hrs = 45 \text{ mins} \ \end{aligned} Total time = 3 hours + 45 mins = 3 hours 45 mins

12) A water tank is two-fifth full. Pipe A can fill a tank in 10 minutes and pipe B can empty in 6 minutes. If both the pipes are open, how long will it take to empty or fill the tank completely

• 1) 6 min to empty
• 2) 7 min to full
• 3) 6 min to full
• 4) 7 min to empty
Ans.   A
Explanation :
There are two important points to learn in this type of question, First, if both will open then tank will be empty first. Second most important thing is, If we are calculating filling of tank then we will subtract as (filling-empting) If we are calculating empting of thank then we will subtract as (empting-filling) So lets come on the question now, Part to emptied 2/5 Part emptied in 1 minute = \begin{aligned} \frac{1}{6} - \frac{1}{10} \= \frac{1}{15} \=> \frac{1}{15}:\frac{2}{5}::1:x \=> \frac{2}{5}*15 = 6 mins \end{aligned}

13) Pipe A can fill a tank in 5 hours, pipe B in 10 hours and pipe C in 30 hours. If all the pipes are open, in how many hours will the tank be filled

• 1) 2.5 hours
• 2) 2 hours
• 3) 3.5 hours
• 4) 3 hours
Ans.   D
Explanation :
Part filled by A in 1 hour = 1/5 Part filled by B in 1 hour = 1/10 Part filled by C in 1 hour = 1/30 Part filled by (A+B+C) in 1 hour = \begin{aligned} \frac{1}{5}+\frac{1}{10}+\frac{1}{30} \= \frac{1}{3} \\end{aligned} So all pipes will fill the tank in 3 hours.

14) Pipes A and B can fill a tank in 5 hours and 6 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled i

• 1) \begin{aligned} 3\frac{9}{5} \end{aligned}
• 2) \begin{aligned} 3\frac{9}{17} \end{aligned}
• 3) \begin{aligned} 3\frac{7}{5} \end{aligned}
• 4) \begin{aligned} 3\frac{7}{17} \end{aligned}
Ans.   B
Explanation :
Net part filled in 1 hour = \begin{aligned} \left(\frac{1}{5}+\frac{1}{6}-\frac{1}{12}\right) \= \frac{17}{60} hrs \= 3\frac{9}{17} \end{aligned}

15) A leak in the bottom of a tank can empty the full tank in 6 hours. An inlet pipe fills water at the rate of 4 litres a minute. When the tank is full, the inlet is opened and due to the leak the tank is empty in 8 hours. The capacity of the tank (in litres)

• 1) 5780 litres
• 2) 5770 litres
• 3) 5760 litres
• 4) 5750 litres
Ans.   C
Explanation :
Work done by the inlet in 1 hour = \begin{aligned} \frac{1}{6}-\frac{1}{8} = \frac{1}{24} \\text{Work done by inlet in 1 min} \= \frac{1}{24}*\frac{1}{60}\= \frac{1}{1440} \=> \text{Volume of 1/1440 part = 4 liters} \ \end{aligned} Volume of whole = (1440 * 4) litres = 5760 litres.

16) A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9 hours. If both the taps are opened simultaneously, then after how much time cistern will get filled

• 1) 7 hours
• 2) 7.1 hours
• 3) 7.2 hours
• 4) 7.3 hours