13.2962963 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
P(a | b) = p(a ∩ b)/p(b) . . . . . a definition worth remembering
0.2 = p(a ∩ b)/0.5 . . . . . . . . fill in the given values
0.2*0.5 = 0.1 = p(a ∩ b)
p(a ∩ b) = 0.1
A postulate isn't required to be proven