The probability is 19.88%.
This is a binomial distribution, because there are two outcomes, the events are independent of each other, and there is a fixed number of trials. The binomial distribution is given by:
[tex](_k^n)p^k(1-p)^{n-k}[/tex]
For this, n is 18; k is 5; p is 0.25:
[tex](_5^{18})(0.25)^5(1-0.25)^{18-5}
\\
\\=\frac{18!}{5!13!}(0.25)^5(0.75)^{13}
\\
\\=8568(0.25)^5(0.75)^{13}
\\=0.1988=19.88%[/tex]
