Subject: mathqa
Question: if p ( a ) = 0.4 , p ( b ) = 0.6 and p ( a ∪ b ) = 0.8 . what is the value of p ( a ∩ b ' ) = ?
Option: 
A: 0.1
B: 0.4
C: 0.3
D: 0.2
Answer: D