One day, Mr P and Mr Q were having a test on logic by Professor White.

On the table in front of them there were 16 cards and both of them knew what they were:

Hearts: A, Q, 4
Spades: J, 8, 4, 2, 7, 3
Clubs: K, Q, 5, 4, 6
Diamonds: A, 5

Then all the 16 cards were turned upside down and reshuffled.

Professor White picked a card randomly and told Mr P the number of the card had and told Mr Q the suit of the card.

Professor White then asked if they could find out what the card was.

Mr P said, "No I can't."
When Mr Q said, "I knew you couldn't."
Having heard this, Mr P said, "ah, now I know."
Then Mr Q said, "oh, so do I now."

Can you tell what card it was by logical deduction?