Solution:
The event follows a geometric distribution with the first success coming after the third trial;
P(X=x)=p(1−p)x−1P(X=x) = p(1-p)^{x-1}P(X=x)=p(1−p)x−1
P(X=3)=0.4(0.6)2P(X =3)=0.4(0.6)^2P(X=3)=0.4(0.6)2
=0.144