Among smartphone users, \(40 \%\) use a case but no screen protector, \(10 \%\) use a screen protector but no case, \(45 \%\) use both a case and a screen protector, and \(5 \%\) use neither a case nor a screen protector. Twenty smartphone users are sampled at random. Let \(X_{1}, X_{2}\), \(X_{3}, X_{4}\) be the respective numbers of users in the four given categories.

a. Find \(P\left(X_{1}=7, X_{2}=3, X_{3}=8ight.\), and \(\left.X_{4}=2ight)\).

b. Find \(P\left(X_{1}=7ight)\).