Beach Coin Standoff

You and your brother are sent to separate rooms and cannot communicate. Each of you independently decides whether to walk out carrying a coin. The coin is biased: it lands heads with probability 0.6.

  • If both of you carry a coin out, your parents flip both, and you go to the beach only if BOTH land heads.
  • If exactly one coin comes out, they flip that one, and you go if it lands heads.
  • If neither of you brings a coin, you stay home.

Before being separated you agreed on one rule: each of you carries the coin out with probability qq, independently of the other. Choose qq to maximise the chance of going to the beach.

What is that maximum probability?

Answer format: round to 3 decimal places.