The total number of methods is A44=24. 1 If A is in the first place, the last three ethyl, propyl and butyl groups are randomly arranged, and A33=6 is arranged; 2 If A is in the second place, there are C2 1=2 kinds of propyl-butyl in the first place, and the third and fourth places are randomly arranged by B and one of the remaining C or D, and A22=2 kinds, so there are 2*2=4 kinds; 3 A ranks third, so the fourth place can only rank second, the first place and the second place are randomly arranged, A22=2 kinds. So the probability that A is not behind B is (6+4+2)/ 12=0.5. This is a detailed step. In fact, even if it is randomly arranged, there are only two situations. A is either before or after B, and the probability is 0.5.