This kind of exercise refers to the probability of exatly "x" successes on "n" repeated trials in an experiment which has a possible outcome.
If the probability of succes on an individial trial is p, then the probability is represented by:
[tex]P=\text{nCx}\cdot p^x\cdot(1-p)^{n-x}[/tex]
Here nCx indicates the number of different combinations of x objects selected from a set of n objects.
Using the given data, we have:
[tex]\begin{gathered} P=18\text{C11}\cdot(0.8^{11})\cdot(1-0.8)^{18-11} \\ P=31824\cdot(8.589\cdot10^{-2})\cdot(1.28\cdot10^{-5})^{} \\ P=0.035 \end{gathered}[/tex]
