# Prepare for Bayes Theorem

Until now we discussed the prior probability of a future event happening and covered the various possibilities. Often we want to know what an event tells us about our circumstances. For example, if our coin keeps on landing on heads, we might suspect that it is weighted.

Before examining that questions, let's look at a simple question. Let's say you know 50% of all your coins are dollars (of any type) and 20% of all your coins are silver dollars. You randomly pick a coin, and - lucky you - its a dollar coin! You don't know if it's silver though.

### Challenge

What is the current probability that your coin is a silver dollar?

Alternatively, you can try out Learneroo before signing up.

• #### Rod

Should not the question be what is the probability that your coin is a silver dollar? Also the distinction between dollars and silver dollars is not clear. Your result implies dollars includes silver dollars.

• #### Jig

I understood that 50% of the coins are dollars and of those 20% are silver, but it can be understood that 50% of the coins are dollars and 20% of the coins are silver dollars.

• #### Learneroo

@Jig, actually it's 20% of the total. See added clarification.

• #### SevK

2/5 should be a correct answer, but it shows as incorrect

• #### Learneroo

The question asks you to enter the answer as an integer or decimal.