You have 2 six-sided dice. One is a normal fair die, while the other has 2 ones, 2 threes, and 2 ﬁves. You pick a die and roll it. Because of some secret magnetic attraction of the unfair die, you have a 75% chance of picking the unfair die and a 25% chance of picking the fair die. If you roll a three, what is the probability that you chose the fair die?
1. **Define the events:**
   - Let \( F \) be the event of choosing the fair die.
   - Let \( U \) be the event of choosing the unfair die.
   - Let \( R_3 \) be the event of rolling a three.

2. **Given probabilities:**
   - \( P(F) = 0.25 \)
   - \( P(U) = 0.75 \)

3. **Calculate the probability of rolling a three with each die:**
   - For the fair die, the probability of rolling a three is \( P(R_3 | F) = \frac{1}{6} \).
   - For the unfair die, the probability of rolling a three is \( P(R_3 | U) = \frac{2}{6} = \frac{1}{3} \).

4. **Use the law of total probability to find \( P(R_3) \):**
   \[
   P(R_3) = P(R_3 | F)P(F) + P(R_3 | U)P(U)
   \]
   Substituting the given values:
   \[
   P(R_3) = \left(\frac{1}{6}\right)\left(\frac{1}{4}\right) + \left(\frac{1}{3}\right)\left(\frac{3}{4}\right)
   \]
   \[
   P(R_3) = \frac{1}{24} + \frac{3}{12} = \frac{1}{24} + \frac{6}{24} = \frac{7}{24}
   \]

5. **Calculate the conditional probability \( P(F | R_3) \) using Bayes' theorem:**
   \[
   P(F | R_3) = \frac{P(R_3 | F)P(F)}{P(R_3)}
   \]
   Substituting the known values:
   \[
   P(F | R_3) = \frac{\left(\frac{1}{6}\right)\left(\frac{1}{4}\right)}{\frac{7}{24}} = \frac{\frac{1}{24}}{\frac{7}{24}} = \frac{1}{7}
   \]

\(\blacksquare\)

The final answer is \( \boxed{ \frac{1}{7} } \)