回复：回复：回复：My daughter's 3rd grade math. Need help...

来源: Jinjing 于 2010-01-18 10:28:05 [] [旧帖] [给我悄悄话] 阅读数 : (332 bytes)
本帖于 2010-04-08 17:50:51 时间, 由版主 于德利 编辑

1*1: ld,dl 2
2*2: lldd,ldld,lddl,dlld,dldl,ddll 6
3*3: lllddd,.................,dddlll 20
4*4: lllldddd,...............,ddddllll70

Permutation: P1=1 p2=2*1 P3=3*2*1,...p8=8*7*6...*1,but only 2 letters,there p4*p4 are the same,so the answer is p8/(p4*p4)=8*7*....*1/(4*3*2*1*4*3*2*1)=70

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