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Senior Manager
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The one who flies is worthy. The one who is worthy flies. The one who doesn't fly isn't worthy
Last edited by spiridon on 15 Oct 2008, 14:38, edited 1 time in total.
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(10x+20y)/30 = 18 10x+20y = 540 ie: x+2y =54 10x+20(y-z) /30-z =14 420-14z = 10x+20y -20z 420+ 6z = 10x+20y,ie: 210+3z = 5x+10y, but 5x+10y = 5(x+2y) ie: 5*54 = 270 210+3z = 270 3z = 60 ,z = 20 not sure though 
Last edited by yezz on 15 Oct 2008, 14:50, edited 1 time in total.
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Senior Manager
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10x+20y = 30*18 x+y = 30 y = 24, x = 6 To find the new 20lb boxes 60+20z = (z+6)14 6z = 84-60 = 24 z = 4 So 20 boxes need to be removed. Where am I going wrong ? 
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Director
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I dont know if my approach is correct or am I missing something? > 18 (Old Mean)*30 (#of boxes) = 540 > 14 (New mean)*30 = 420 Difference = 120. Since you need to reduce the 20p boxes, 120/20 = 6 boxes needs to be removed so that he mean reaches 14? I'm confused 
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Senior Manager
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pawan203 wrote: I dont know if my approach is correct or am I missing something? > 18 (Old Mean)*30 (#of boxes) = 540 > 14 (New mean)*30 = 420 Difference = 120. Since you need to reduce the 20p boxes, 120/20 = 6 boxes needs to be removed so that he mean reaches 14? I'm confused your reducement will bring number of boxes to 30-6=24 and if u divide 420 with 24 u get 17.5 not 14
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The one who flies is worthy. The one who is worthy flies. The one who doesn't fly isn't worthy
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Director
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spiridon - thanks, i understand. > (10x+20y)/30 = 18 > 10x+20y=540.....let this be (a) z = number of boxes to be removed > (10x+20(y-z))/30-z = 14 > 10x+20y-20z = 14*(30-z) > 10x+20y-20z = 420-14z > 540-420 = 20z-14z > 120 = 6z z = 20 I dont know if this is correct D?
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Senior Manager
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OA is 20 and i got tricked and selected 6 as well!
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The one who flies is worthy. The one who is worthy flies. The one who doesn't fly isn't worthy
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Retired Moderator
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How long did it take for you guys to do this problem? 5+ minutes for me
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Senior Manager
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very good question at first it took less then a minute but produced wrong answer second attempt (knowing the a) still took a while idk how ppl manage to answer all questions on the gmat
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The one who flies is worthy. The one who is worthy flies. The one who doesn't fly isn't worthy
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Director
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Its took me over 5 mins, as you can see I was confused - posted 2 answers.
Good question.
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Here's how to do it quickly.
This is a weighted average of 10 and 20. Since 18 is 2/10 of the total distance from 20, this means we use 8/10*30=24 boxes that are 20lbs and 6 that are 10 lbs. The new weighted average of 14 is closer to 10 than 20 so now we must use more 10lb boxes than 20lb boxes. Answer D is the only answer that makes sense.
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Manager
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Excellent question.
It took me around 5 min as well. almost gave up, but got it.
20 is the answer as well for me.
Where did you get this question from.
Thanks again.
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Senior Manager
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LiveStronger wrote: 10x+20y = 30*18 x+y = 30 y = 24, x = 6 To find the new 20lb boxes 60+20z = (z+6)14 6z = 84-60 = 24 z = 4 So 20 boxes need to be removed. Where am I going wrong ? So the answer was 20 afterall  spiridon: I didn't see 20 as part of the initial list, did you change it later ?
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Senior Manager
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LiveStronger wrote: LiveStronger wrote: 10x+20y = 30*18 x+y = 30 y = 24, x = 6 To find the new 20lb boxes 60+20z = (z+6)14 6z = 84-60 = 24 z = 4 So 20 boxes need to be removed. Where am I going wrong ? So the answer was 20 afterall spiridon: I didn't see 20 as part of the initial list, did you change it later ? yeah made a typo sorrry
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The one who flies is worthy. The one who is worthy flies. The one who doesn't fly isn't worthy
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spiridon wrote: 4
6
10
20
24 I get 20... equation is as follows (10x+20(30-x))/30=18 10x+600-20x=30*18 30*20-10x=30*18 30(20-18)=10x; 30(2)=10x; x=6, so there are 24 20-lb boxes now.. (10*6+20(24-x))/(30-x)=14 60+20*24-20x=14(30-x) 30*2-30*14+24*20=6x -30(12)+12(2*20)=6x; 12(-30+40)=6x; 12(10)=6x; x=2*10 or 20... 20 boxes need to be moved
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