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A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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20 Nov 2012, 18:00
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A cylindrical water tower with radius 5 m and height 8 m is 3/4 full at noon. Every minute, .08π m3 is drawn from tank, while .03π m3 is added. Additionally, starting at 1pm and continuing each hour on the hour, there is a periodic drain of 4π m3. From noon, how many hours will it take to drain the entire tank? A. 20 2/7 B. 20 6/7 C. 21 D. 21 3/7 E. 22
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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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20 Nov 2012, 21:07
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danzig wrote: With 3 m3 remaining, after another 3/7 hours, only 3(0.5) = 1.5 m3 will be drained. So the tank will not actually be empty until 22 hours, when the periodic draw empties the remainder." I don't understand the OE does this: 3(0.5) = 1.5 m3 :S !
Thanks! Every minute, .05 m^3 is drawn from the tank. In one hour, water drawn from the tank will be 60*.05 = 3 m^3. Since at the end of 21st hr, 3 m^3 water is left in the tank, at the end of the 22nd hour, the entire 3 m^3 water will be drawn and there will be nothing left to drain out in periodic draining. Further, in 3/7 hrs, (3/7)*(60)*(.05) = 9/7 m^3 water is drawn I do not know why they write 3(0.5) = 1.5
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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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26 Nov 2012, 05:58
at 12: 3/4 * 200 = 150 pie m3 at 12  1: .08 + .03 = .05 * 60 = 3pie m3 at 1  147 pie m3 ===> 3 pie m3 + 4 pie m3 = 7 pie m3 ==> so in hour effectively 7 so it takes 21 hours for 147.
thus 21 (1 thereon)+ 1 (121) = 22 hours.



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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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17 Dec 2012, 03:24
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This one got me, it is so hard to pay attention to every detail and think so much. I initially went with D. So D is wrong why why?
Alright so initial volume = (3/4)×∏×5²×8 = 150∏ Relative Drain/ min = .08∏  .03∏ = .05∏ m³/min drain Relative drain / hour = .05∏×60 = 3∏ m³/ hr
Every one hour starting from 1pm, 4∏ m³ of water is drained. It means that only at the hour the water is drained and NOT “in that 1 hour“
So after 1 hr the relative drain would be 3∏ + 4∏ = 7∏m³ of water drain
What i did initially was formed an equation 150∏ = 7∏×n (n is the number of hrs) so ended up with 21 3/7. This wrong
Look at this way after 21 hrs the amount of water drain will be 21×7∏ = 147∏ m³ Left over water in the tank after 21 hrs = 3∏ m³ From above we know that it take 1 more hour to drain that 3∏ m³.
So ans is 22hrs



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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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17 Dec 2012, 03:57
Another easy approach to this question is: Volume to be drained= 150pi Every minute, the volume that drains is : (0.08pi0.03pi) metre cube or 0.05pi metre cube. Every hour 4pi metre cube also drains out, but remember this starts 1 hour later. We can set up an equation here: \(0.08*number[m]\)of\(\)minutes\(\)in\(\)y\(\)hours\(+\)4(y1)=150[/m] or \(0.08*60*y + 4y=154\) or \(7y=154\) Therefore y=\(22 hours\) I found this approach rather easier.
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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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07 Aug 2014, 09:58
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12 noon: 150π m3
For every minute there is a net drain of 0.08π0.03π=0.05π m3. So for every hour=0.05*60=3π m3/hour This means at 1pm there is 150π3π=147π m3
And from 1pm there is an additional drain of 4π m3 so net drain= 3π+4π=7π m3/hour So the time taken to drain 147π m3 is 147π/7π=21 hours. and lets not forget the 1 hour between 12 and 1pm.
Therefore, total time taken is 21+1=22 hours



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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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21 Aug 2014, 11:58
3/4 of 200pi = 150pi.
0.08pi removed & 0.03pi added for each minute. for hours 4.8pi removed & 1.8pi added. consolidated will be 3.0pi removed for each hour. Note: This starts at noon.
147pi.
starting from 1pm for each hour 4 pi is removed from the total. (along with the regular 3.0pi) , it will be 7pi removed from 1 pm .
147pi/7pi = 21 hours.
Adding the hour between 12 to 1 pm  21+1 = 22 hours.



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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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22 Apr 2015, 01:41
Marcab wrote: Another easy approach to this question is:
Volume to be drained= 150pi Every minute, the volume that drains is : (0.08pi0.03pi) metre cube or 0.05pi metre cube. Every hour 4pi metre cube also drains out, but remember this starts 1 hour later.
We can set up an equation here: \(0.08*number[m]\)of\(\)minutes\(\)in\(\)y\(\)hours\(+\)4(y1)=150[/m] or \(0.08*60*y + 4y=154\) or \(7y=154\) Therefore y=\(22 hours\)
I found this approach rather easier. hi, I have not understood why we have used .08 instead of .05(net drainage) please help



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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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22 Apr 2015, 03:56
ssriva2 wrote: Marcab wrote: Another easy approach to this question is:
Volume to be drained= 150pi Every minute, the volume that drains is : (0.08pi0.03pi) metre cube or 0.05pi metre cube. Every hour 4pi metre cube also drains out, but remember this starts 1 hour later.
We can set up an equation here: \(0.08*number[m]\)of\(\)minutes\(\)in\(\)y\(\)hours\(+\)4(y1)=150[/m] or \(0.08*60*y + 4y=154\) or \(7y=154\) Therefore y=\(22 hours\)
I found this approach rather easier. hi, I have not understood why we have used .08 instead of .05(net drainage) please help Hi ssriva2, You are right, it has to be 0.05, which is net drainage and not 0.08. The equation writes 0.08 but solves using 0.05. Assuming 0.08 in the equation will not give the right answer. Regards Harsh
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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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22 Jun 2015, 08:03
danzig wrote: A cylindrical water tower with radius 5 m and height 8 m is 3/4 full at noon. Every minute, .08π m3 is drawn from tank, while .03π m3 is added. Additionally, starting at 1pm and continuing each hour on the hour, there is a periodic drain of 4π m3. From noon, how many hours will it take to drain the entire tank?
A. 20 2/7 B. 20 6/7 C. 21 D. 21 3/7 E. 22
I agree with the OA. After 21 hours, 147π m3 will be drained. So, there are 3π m3 that have not been drained. Because the constant drain draws 3π m3 per hour, we will have to wait until the 22th hour. Probably, the periodic drain will draw little water in the 22th hour. Please, confirm whether my reasoning is Ok.
In this sense, I don't understand this part of the OE: "Had we divided 150/7, we'd land on , but we have to consider how the 3/7 remainder actually leaves the tank.
Now we have to deal with remainders. With 3 m3 remaining, after another 3/7 hours, only 3(0.5) = 1.5 m3 will be drained. So the tank will not actually be empty until 22 hours, when the periodic draw empties the remainder." I don't understand the OE does this: 3(0.5) = 1.5 m3 :S !
Thanks! Volume of the cylinder is 150pi At 1 pm the volume is 147pi (the net drain is 3pi) Then onwards, the net drain is 7pi per hour. Setting AP we get 21 hours. Therefore, total time taken is 22 hours.



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A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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17 Jul 2016, 06:26
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danzig wrote: A cylindrical water tower with radius 5 m and height 8 m is 3/4 full at noon. Every minute, .08π m3 is drawn from tank, while .03π m3 is added. Additionally, starting at 1pm and continuing each hour on the hour, there is a periodic drain of 4π m3. From noon, how many hours will it take to drain the entire tank?
A. 20 2/7 B. 20 6/7 C. 21 D. 21 3/7 E. 22 AT 12 O' CLOCK ) Water at the tank =\(π*5^2*8*\frac{3}{4} = 150π\) Water Removed =\(\frac{0.08π}{min}\); Water Added=\(\frac{0.03π}{min}\) ==> net effect = Loss of \(\frac{0.05π}{min}\) AT 1 PM ) Water at the tank = 150π 0.05π*60 ==>150π3π=147π ( TIME TAKEN =1 hour)Now starting at 1 PM , in addition to 3π another 4π of water is removed every hour so Net Loss of water ever hour = 3π+4π =7π Let t be the time taken by tank to completely empty 147π7π*t=0 7π*t=147π t=\(\frac{147π}{7π}\)= 21TOTAL TIME = 21 hours + 1 HOUR (FROM 12 O CLOCK TO 1 O CLOCK) = 22ANSWER IS E
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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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17 Jul 2016, 10:28
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danzig wrote: A cylindrical water tower with radius 5 m and height 8 m is 3/4 full at noon. Every minute, .08π m3 is drawn from tank, while .03π m3 is added. Additionally, starting at 1pm and continuing each hour on the hour, there is a periodic drain of 4π m3. From noon, how many hours will it take to drain the entire tank?
A. 20 2/7 B. 20 6/7 C. 21 D. 21 3/7 E. 22 Volume of water present in the tank= 3/4 π r^2h= 3/4π *5*5*8= 150π Every minute after noon till 1:00 pm the net water that is drained out was= .08π m3 .03π m3 = .05 π cubic meter In one hr from noon to 1:00 pm, total water loss was = .05 π *60= 3 π cubic meter At 1:00 pm 150π3π= 147π water is remaining that has to be drained out. In addition to 3π, 4π water is also draining out of the tank, making total 7π water draining out after 1:00 pm 147π water will be drained out in 147π/7π= 21 hrs after 1 pm Add 1 hr to this as water was drained out from noon to 1:00 pm, making total 21+1= 22 hrs E is the answer
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Re: A cylindrical water tower with radius 5 m and height 8 m is [#permalink]
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29 Apr 2017, 10:36
danzig wrote: A cylindrical water tower with radius 5 m and height 8 m is 3/4 full at noon. Every minute, .08π m3 is drawn from tank, while .03π m3 is added. Additionally, starting at 1pm and continuing each hour on the hour, there is a periodic drain of 4π m3. From noon, how many hours will it take to drain the entire tank?
A. 20 2/7 B. 20 6/7 C. 21 D. 21 3/7 E. 22 Let, the time taken was 't' hours [ say, if the time is 1.5 hrs the periodic drain would be 1 time. Similarly, if time is 2 hrs periodic drain would be 2 times & for 15.36hrs it would be 15 times & so on.. So, no. of periodic drain is clearly an integer] . Lets assume its 'n'. Now, pie x 5^2 x (8) x 3/4 + (0.08pie + 0.03pie) x t x 60  n x 4 x pie = 0 =>(150 3t)/4 = n So, substituting the values in the options, we find only 22 (option E) gives us n=21 (integer) rest all give us decimal no.s . Hence, Answer is option E.




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