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.

Find P(X1=7, X2=3, X3=8, X4=2).