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

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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• 回复：回复：回复：回复：My daughter's 3rd grade math. Need help... -JIAYANGGUIZI2- ♂ (137 bytes) () 01/18/2010 postreply 10:45:37