Model 3 Opening both taps and leaks Section-Wise Topic Notes With Detailed Explanation And Example Questions

MOST IMPORTANT quantitative aptitude - 3 EXERCISES

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The following question based on pipes & cisterns topic of quantitative aptitude

Questions : A tank can be filled by two pipes in 20 minutes and 30 minutes respectively. When the tank was empty, the two pipes were opened. After some time, the first pipe was stopped and the tank was filled in 18 minutes. After how much time of the start was the first pipe stopped?

(a) 12 minutes

(b) 10 minutes

(c) 5 minutes

(d) 8 minutes

The correct answers to the above question in:

Answer: (d)

Let the first pipe be closed after x minutes

$x/20 + 18/30$ = 1

$x/20 = 1 - 18/30 = 1 - 3/5 = 2/5$

$x = 2/5 × 20$ = 8 minutes

Using Rule 8,
Two taps A and B can fill a tank in x hours and y hours respectively. If both the pipes are opened together, then the time after which pipe B should be closed so that the tank is full in t hours
Required time = $[y(1 –{t/x})]$ hours

Here, x = 20, y = 30, t = 18

Required time = $[x(1 –{t/y})]$

[Since, first pipe is closed]

= $[20(1 - 18/30)] = 20 × 12/30$ = 8 minutes

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Read more opening both taps and leaks Based Quantitative Aptitude Questions and Answers

Question : 1

A water tap fills a tub in ‘p’ hours and a sink at the bottom empties it in ‘q’ hours. If p < q and both tap and sink are open, the tank is filled in ‘r’ hours; then

a) r = p - q

b) r = p + q

c) $1/r = 1/p + 1/q$

d) $1/r = 1/p - 1/q$

Answer: (d)

Since, P < q,

On opening pipe and sink together,

Part of the tub filled in 1 hour

= $1/P - 1/q$

Clearly, $1/P - 1/q = 1/r$

Question : 2

Two pipes A and B can fill a tank in 6 hours and 8 hours respectively. If both the pipes are opened together, then after how many hours should B be closed so that the tank is full in 4 hours?

a) $8/3$ hrs

b) 2 hrs

c) $2/3$ hrs

d) 1hrs

Answer: (a)

Part of the tank filled in 4 hours by pipe A = $4/6 = 2/3$

Remaining part = ${1 - 2}/3 = 1/3$

Time taken by pipe B in filling $1/3$ part = $8/3$ hours

Using Rule 8,

Here, x = 6, y = 8, t = 4

Required time = $[y(1 –{t/x})]$ hours

= $[8(1 - 4/6)]$ hours = $8/3$ hours

Question : 3

A tank can be filled with water by two pipes A and B together in 36 minutes. If the pipe B was stopped after 30 minutes, the tank is filled in 40 minutes. The pipe B can alone fill the tank in

a) 90 minutes

b) 75 minutes

c) 45 minutes

d) 60 minutes

Answer: (a)

Let the pipe B fill the tank in x minutes.

Part of the tank filled by pipes A and B in 1 minute = $1/36$

Part of the tank filled by pipe A in 1 minute

= $1/36 - 1/x$

According to the question,

30 × $1/x + 40(1/36 - 1/x) = 1$

$30/x + 10/9 - 40/x$ = 1

$40/x - 30/x = 10/9 - 1$

$10/x = 1/9 ⇒ x = 90$ minutes

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