Let's review the initial probabilities from the basic probability questions:
- Initial probability of coin being weighted and landing on heads: (.1*.8 = .08)
- Initial probability of coin being normal and landing on heads: (.9*.5 = .45)
- Combined probability of getting heads: (.08+.45= .53)
Now, for the grand question: You pick a coin at random and flip it, and it lands on heads. What is the probability that the coin is weighted?
In the last question, you just looked at the fraction of dollar coins that were silver. In this question you need to look at a different fraction to get the probability that the coin is weighted.