Richard rolls a fair six-sided die repeatedly until he rolls his twentieth prime number or his second even number. Compute the probability that his last roll is prime.
1. **Define the problem and probabilities:**
   - Richard rolls a fair six-sided die repeatedly until he rolls his twentieth prime number or his second even number.
   - The prime numbers on a six-sided die are 2, 3, and 5.
   - The even numbers on a six-sided die are 2, 4, and 6.
   - The probability of rolling a prime number (2, 3, or 5) is \(\frac{3}{6} = \frac{1}{2}\).
   - The probability of rolling an even number (2, 4, or 6) is \(\frac{3}{6} = \frac{1}{2}\).
   - The probability of rolling a non-even prime (3 or 5) is \(\frac{2}{6} = \frac{1}{3}\).
   - The probability of rolling an even prime (2) is \(\frac{1}{6}\).
   - The probability of rolling a composite even (4 or 6) is \(\frac{2}{6} = \frac{1}{3}\).

2. **Define the recursive probabilities:**
   - Let \(E(m, n)\) be the probability Richard gets to the point where he has rolled \(m\) primes and \(n\) evens.
   - \(E(0,0) = 1\) (initial state).
   - \(E(n,0) = \left(\frac{2}{5}\right)^n\) (probability of rolling \(n\) non-even primes without rolling any even number).
   - \(E(0, 1) = \frac{2}{5}\) (probability of rolling one even number without rolling any primes).

3. **Establish the recursion:**
   - The recursion for \(E(m, n)\) is:
     \[
     E(m, n) = \frac{2}{5} E(m, n-1) + \frac{1}{5} E(m-1, n-1) + \frac{2}{5} E(m-1, n)
     \]
   - For \(E(n, 1)\):
     \[
     E(n, 1) = \frac{2^{n+1}}{5^{n+1}} + \frac{2^{n-1}}{5^n} + \frac{2}{5} E(n-1, 1)
     \]
   - Simplify using \(F(n) = \frac{5^n}{2^n} E(n, 1)\):
     \[
     F(n) = \frac{9}{10} + F(n-1)
     \]
   - This is an arithmetic sequence with \(F(0) = \frac{2}{5}\):
     \[
     F(n) = \frac{9}{10}n + \frac{2}{5}
     \]

4. **Calculate \(F(19)\):**
   - Using the arithmetic sequence formula:
     \[
     F(19) = \frac{9}{10} \cdot 19 + \frac{2}{5} = \frac{171}{10} + \frac{2}{5} = \frac{171}{10} + \frac{4}{10} = \frac{175}{10} = 17.5
     \]
   - Therefore:
     \[
     E(19, 1) = \frac{2^{19}}{5^{19}} \cdot 17.5 = \frac{7 \cdot 2^{18}}{5^{18}}
     \]

5. **Calculate \(E(19, 0)\):**
   - Using the formula for \(E(n, 0)\):
     \[
     E(19, 0) = \left(\frac{2}{5}\right)^{19}
     \]

6. **Calculate the final probability:**
   - The probability that the last roll is prime:
     \[
     \frac{3}{5} \cdot \frac{2^{19}}{5^{19}} + \frac{3}{5} \cdot \frac{7 \cdot 2^{18}}{5^{18}} = \frac{3}{5} \cdot \frac{2^{19}}{5^{19}} \left(1 + \frac{35}{2}\right)
     \]
   - Simplify:
     \[
     \frac{3}{5} \cdot \frac{2^{19}}{5^{19}} \cdot \frac{37}{2} = \frac{3 \cdot 2^{18} \cdot 37}{5^{20}}
     \]

The final answer is \(\boxed{\frac{2^{18} \cdot 3 \cdot 37}{5^{20}}}\)