回複：回複：回複：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