Step 1: Use Bayes' Theorem to find the probability that the signal is green, given the signal received at station B.
Step 2: The probability of receiving a green signal correctly at station B is \( P(\text{green received}) = \frac{4}{5} \times \frac{3}{4} \), and the probability of receiving a red signal correctly is \( P(\text{red received}) = \frac{1}{5} \times \frac{3}{4} \).
Step 3: Use Bayes' formula to find the conditional probability, which gives \( \frac{20}{23} \).
Final Answer:
\[
\boxed{\frac{20}{23}}
\]