Just a note in case it helps anyone in the future...
The question states:
'Assuming that no students qualified in April 2019...calculate the probability of ruin when the October 2019 exam results are released'
I initially attempted to answer this using the conditional probability formula:
P(A|B) = P(A & B)/ P(B)
= P( ruin | no students qualified in April 2019 ) / P(no students qualified in April 2019)
...and calculating P(no students qualified in April 2019) using the binomial distribution.
After seeing the solutions I realised this was the wrong thing to do...
When we are told 'assume that no students qualified in April 2019', this is telling us:
P(no students qualified in April 2019) = 1
