On the Venn diagram shade the region $A' \cap B'$.

Find ${\text{P}}(A'|B')$.

     A1

[1 mark]

$P(A'|B') = \frac{{P(A' \cap B')}}{{P(B')}}$    (M1)

$P(B') = 0.1 + 0.2 = 0.3$    (A1)

$P(A' \cap B') = 0.1$    (A1)

$P(A'|B') = \frac{{0.1}}{{0.3}} = \frac{1}{3}$    A1

[4 marks]

Part (a) was well done.

In part (b) some candidates were unable to write down the conditional probability formula. Some then failed to realise that part (a) was designed to help them work out $P(A' \cap B')$ and instead incorrectly assumed independence.