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....

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 X₁, X₂, X₃, X₄
be the respective numbers of users in the four given categories. a) Find P(X₁
= 7, X₂
= 3, X₃
= 8, and X₄
= 2). b) Find P(X₁
= 7).