Each accident can occur in any of the 7 days in a week, therefore, the probability of occurrence of an accident in a week is, P(a)=1/7
There are a total of 7 accidents, therefore the sample space is (1/7)^7
Now, all these accidents are occurring on the same day, so now, we must choose 1 day out of 7 days, ie, 7C1
The required probability is (7C1*((1/7)^7)), ie (1/7)^6