ATTENTION: Unfortunately, a small mistake has crept in here: in the third column of the slot machine, the letters should be J and Q instead of B and D. Please note.
This seems to be a classic 'complete the sequence' puzzle. It seems that Charlie still remembered it from his recruitment test many years ago…
The values on the cards play an important role. The cards in the first and third rows seem to always jump two. Now if you also combine the colours of the cards…
It can only be the 5 of spades. For the colour black must follow black (club) and red (diamond). And as the ace counts as a 1 in this case and the difference is still 2 it can only be the 5. The symbol on the back of the card shows you the next letter. A little hint: it is part of the flag of the next country.