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Consider 20 identical pipes that supply water to a tank. What is the

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Consider 20 identical pipes that supply water to a tank. What is the  [#permalink]

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New post 31 Aug 2014, 03:16
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Consider 20 identical pipes that supply water to a tank. What is the minimum number of pipes that has to be kept open if one needs to fill the tank within 30 minutes?


1) When 8 pipes are kept open, the tank gets filled 20 minutes sooner than when 6 pipes are kept open.

2) If all the 20 pipes are kept open, the tank gets filled in less than 15 minutes.


Source: 4gmat


Please help. DS seems to be my greatest weakness :cry:
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Re: Consider 20 identical pipes that supply water to a tank. What is the  [#permalink]

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New post 31 Aug 2014, 05:52
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alphonsa wrote:
Consider 20 identical pipes that supply water to a tank. What is the minimum number of pipes that has to be kept open if one needs to fill the tank within 30 minutes?


1) When 8 pipes are kept open, the tank gets filled 20 minutes sooner than when 6 pipes are kept open.

2) If all the 20 pipes are kept open, the tank gets filled in less than 15 minutes.


Source: 4gmat


Please help. DS seems to be my greatest weakness :cry:


Let x be the minutes a pipe takes to fill the tank.
Therefore rate per pipe= 1/x

statement 1 :
When 8 pipes are kept open, the tank gets filled 20 minutes sooner than when 6 pipes are kept open.

Let m be the minutes 6 pipes take to fill the tank
8(m-20)/x=6m/x
8m-160=6m
2m=160
m=80

6 pipes take 80 minutes to fill the tank. we need to find minimum number of pipes to fill the tank within 30 minutes

6 ----------- 80
?------------ 30

?= 6*80/30
=16 pipes

16 pipes will fill the tank in 30 minutes. so we need atleast 17 pipes to fill the tank within 30 minutes.
Statement is sufficient

statement 2:

If all the 20 pipes are kept open, the tank gets filled in less than 15 minutes.

Doesn't help. 20 pipes may take 5 minutes to fill the tank or may take 14 minutes to do so. Answer will vary as per actual time taken.
Statement is not sufficient.

Hence Ans= A
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Re: Consider 20 identical pipes that supply water to a tank. What is the  [#permalink]

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New post 31 Aug 2014, 08:12
alphonsa wrote:
Consider 20 identical pipes that supply water to a tank. What is the minimum number of pipes that has to be kept open if one needs to fill the tank within 30 minutes?


1) When 8 pipes are kept open, the tank gets filled 20 minutes sooner than when 6 pipes are kept open.

2) If all the 20 pipes are kept open, the tank gets filled in less than 15 minutes.


Source: 4gmat


Please help. DS seems to be my greatest weakness :cry:


Let the total capacity of tank be W ltr
Let each pipe suppply x ltr/min
Let k be minimum number of pipes required to fill tha tank within 30 mins (W/kx =< 30) --- equation 1.

1) W/8x = W/6x - 20
=> W/24x = 20
=> W/x = 480
put this in equation 1.
480/k =< 30
k >= 16
1 is sufficient.

2) W/20x < 15
W/x < 300
Also, W/x =< 30k
combining these would give multiple answers.

Hence, A.
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Re: Consider 20 identical pipes that supply water to a tank. What is the  [#permalink]

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