一道GMAT数学题Q19:Each of the 30 boxes in a certain shipment weig

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一道GMAT数学题
Q19:
Each of the 30 boxes in a certain shipment weighs either 10 pounds or 20 pounds,and average (arithmetic mean) weight of the boxes in the shipment is 18 pounds.If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes,how many 20-pound boxes must be removed?
A.4
B.6
C.10
D.20
E.24

D.10x+20y/30=18 x+y=30 得 x=6 y=24
10m+20n/m+n=14 得2m=3n 因为 m=x=6 所以 n=4
y-n=24-4=20

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