To get the probability of selecting a person who is a democrat and is favour to the issue, let us first calculate for the portions in each party who is favour on the issue.
p (rep & favour) = 0.4 * 0.3 = 0.12
p (dem & favour) = 0.6 * 0.7 = 0.42
Therefore the total portions who are favour is:
p (favour) = 0.12 + 0.42 = 0.54
Hence the probability is:
P (dem & favour) = 0.42 / 0.54 = 0.7778 = 0.78
Therefore there is about 78% probability that the person selected who is favour to the issue is a democrat.
