Explanation:
remember, a probability is ultimately always
desired cases / total possible cases
what are the total possible cases for our question here ?
all registered voters in 2009 :
640 + 960 + 320 = 1920
the desired cases (not republican) out of all these 1920 possibilities are
640 + 320 = 960
so, the probability for a registered voter to not be republican was
960 / 1920 = 1/2 = 0.5