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AJB77
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A water tank is supplied by two pipes, A and B. A alone can 15 Jun 2005, 19:28
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A water tank is supplied by two pipes, A and B. A alone can fill the tank in 36 minutes while B alone can fill it in 45 minutes. There is a discharge pipe C which can empty the tank in 30 minutes if opened alone. A and B are opened first and 7 minutes later C is opened. How many minutes after C is opened does the tank reach capacity?

a) 44
b) 42
c) 39
d) 52
c) 46



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A water tank is supplied by two pipes, A and B. A alone can 15 Jun 2005, 19:40
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7(1/45 + 1/36) + x(1/45 + 1/36 -1/30) = 1

guess is x = 39, maybe
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ywilfred
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A water tank is supplied by two pipes, A and B. A alone can 15 Jun 2005, 19:45
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A fills the tank at a rate of 1/36 per minute, B fills the tank at a rate of 1/45 per minute
A and B together fills the tank at a rate of (1/36+1/45) = 1/20 tank per minute

After 7 minutes, 7/20 of the tank would be filled

C discharges water at a rate 1/30 per minute, so the rate of water filling the tank is 1/20-1/30 = 1/60 per minute.
Need to fill 13/20 of the tank, this would take 13/20 * 60 = 39 minutes

C it is.
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HIMALAYA
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A water tank is supplied by two pipes, A and B. A alone can 15 Jun 2005, 20:13
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I also got 39 minuets..........

sparky
7(1/45 + 1/36) + x(1/45 + 1/36 -1/30) = 1
guess is x = 39, maybe
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sweet one.
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HongHu
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A water tank is supplied by two pipes, A and B. A alone can 15 Jun 2005, 22:03
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AJB77
A water tank is supplied by two pipes, A and B. A alone can fill the tank in 36 minutes while B alone can fill it in 45 minutes. There is a discharge pipe C which can empty the tank in 30 minutes if opened alone. A and B are opened first and 7 minutes later C is opened. How many minutes after C is opened does the tank reach capacity?

a) 44
b) 42
c) 39
d) 52
c) 46
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The following can be used if you are short of time, or as a sanity check after your calculation:
B alone only need 45 min. C is a little more than cancel out A but A and B has been working together to fill about 1/3 of the tank. So 46 and 52 is definitely out and 44 too. For B to fill the rest of 2/3 it will need appr 30 minutes. 39 would be a good guess.
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A water tank is supplied by two pipes, A and B. A alone can 16 Jun 2005, 00:57
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Rate that the tank is filled by both = 1/36+1/45=1/20
7/20 + (1/20-1/30)x = 1
7/20 + x/60 = 1
x/60 = 13/20
x=39
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mandy
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A water tank is supplied by two pipes, A and B. A alone can 21 Jun 2005, 16:26
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cloudz9
Rate that the tank is filled by both = 1/36+1/45=1/20
7/20 + (1/20-1/30)x = 1
7/20 + x/60 = 1
x/60 = 13/20
x=39
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:) plz Cloudz just a small question why do you equate the equation to 1 thanks!
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AJB77
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A water tank is supplied by two pipes, A and B. A alone can 21 Jun 2005, 20:39
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Mandy,

In fraction/rate problems, you assume 1 to be represent the total work that is needed. Here it is the filling of the tank.

For example, if pipe A alone can fill a tank in 36 minutes then how much of the tank does pipe A fill in 1 minute?

Answer = 1/36 of the tank.

You may find it easier to do by assuming the tank's capacity to be X gallons where X is sthe LCM of the various minute numbers given. In this case, assume that the tank's capacity is 180 gallons (180 is the LCM of 36, 45 and 30)

Pipe A fills 180/36 = 5 gal/min
Pipe B fills 180/45 = 4 gal/min => A and B together fill 9 gal/min
Pipe C empties 180/30 = 6 gal/min

A and B are opened for 7 minutes => They fill 7 * 9 = 63 gallons

Remaining = 117 gal

Then C is opened.

From then on the tank is filled at (5+4 - 6) = 3 gal/min

How many more minutes are needed?

117/3 = 39 minutes



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This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
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