(c) is done pretty much the same as (a) and (b). Find the probability that the number of boats requested is greater than 20: P(x> 20.5).
Notice that I used the "integer correction". In the Poisson distribution, x must be an integer. In the Normal distribution it can be any real number. We need to use x in the normal distribution that would round to the given integer. Did you do that for (a) and (b)? (a) would be P(x< 5.5), (b) is P(9.5< x< 10.5) and (c) is P(x> 20.5).