2013gre考試:3月gre數(shù)學疑難解答(三)

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    1.從2 3 4 5 6 7 8 9中取數(shù)字組成三位數(shù)號碼,assign to 330 employee, 數(shù)字不重復(fù), 問多少號碼unassigned?
     :P(3,8)=(8*7*6)=336
    336-330=6
    2.一個工作W,X做要比Y做少兩天,X,Y一起做4/3W要3天,求X單獨做2W要幾天?
     :令W/X=x,W/Y=y,則x+2=y (1)
    (4/3W)/(W/x+W/y)=3 (2)
    兩式連立化去y,即可解得x