A can come between 8 to 10 , which is 120 min duration . A can come on any instant from this 120 min window .
B can come between 9 to 11 , which is 120 min duration . B also can choose any instant for coming from this 120 min window
so tota possible cases of coming of A nd B =120*120
Now consider the case where B is coming early to A . here common window of 60 min which is from 9 to 10 .
we can choose two time instant from this window by 60C2 ways .
so probability is = 60c2/120*120 = 60*59
--------------= 59/480
2*120*120