Dice 問題還沒完， 請大家看為什麽我的算法不對？ 我得向小平家交代：）

i first calculate the probability a<=b, 

(6+5/6+4/6+3/6+2/6+1/6)* 1/6 = 21/36

so it is same probablity for b<=c, c<=d

so  (21/36) *(21/36)*(21/36) or  (21/36)^3 is the answer 

where is wrong?