Problem: The economics department has determined that there is a 30% chance that economy will be strong. Further inquiries reveal that if the stock market is up over the next three months, there is a 50% chance that the economy will be strong, bit if it is down, the chances are only 20%.
I made a probabilities table:
strong weak Total
up 50% 50% 100%
down 20% 80% 100%
Total 30% 70% 100%