The probability that a passenger does show up is 0.8
The probability that 9 tickets are sold and all passengers 9 show up =
This situation will cause a rejected passenger.
If 10 tickets are sold and all 10 passengers show up, 2 passengers will be rejected.
If 10 tickets are sold and any 9 of these 10 passengers show up, 1 passenger gets rejected.
There are ways that 9 of the 10 can show up, which is the same as saying that there are 10 ways 1 passenger doesn't show up.
You must add all probabilities together to find the probability of rejection.