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wonder_gmat
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From a cask of milk containing 30 gallons, 6 gallons are 23 Nov 2003, 08:36
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From a cask of milk containing 30 gallons, 6 gallons are drawn out and the cask is filled up with water. If the same process is repeated a second, then a third time, what will be the number of gallons of milk left in the cask?

(1) 0.512
(2) 12
(3) 14.38
(4) 15.36


I found this question another forum. Kinda interesting question... makes you stop and think for a second.



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From a cask of milk containing 30 gallons, 6 gallons are 23 Nov 2003, 10:48
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After first removal: 24/0
After first replacement 24/6
After second removal .8(24)/.8(6) 19.2/4.8
After second replacement 19.2/10.8
After third removal .8(19.2)/.8(10.8) 15.36/ whatever...

Easier still, .8^2 * 24, but it didn't occur to me at the time...
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From a cask of milk containing 30 gallons, 6 gallons are 27 Nov 2003, 17:21
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Amount of milk left/Original amount of milk = (1- amount removed/original amount)^3

The answer is 15.36
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From a cask of milk containing 30 gallons, 6 gallons are 01 Dec 2003, 23:50
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gmatblast
Amount of milk left/Original amount of milk = (1- amount removed/original amount)^3

The answer is 15.36


this approach is the best.
my solution: count concentration of milk in the mix
initianlly C=100%, and each time we remove 6/30=1/5 of the mix and add the same amount of water. Therefore, the concentration decreases each time by 20%.

C=100*0.8*0.8*0.8=51.2%
Milk=30*51.2%=15.36



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