Number of possible outcomes = 3+4+2+1 = 10
P(Chocolate cake) = 4/10 = 0.4
Let the events:
A = first customer purchases a chocolate cake
B = second customer purchases a chocolate cake
Since the two events are independent,
P(A and B) = P(A) * P(B) = 0.4*0.4 = 0.16
Answer = 0.16