I think you're confusing yourself about what's happening here (or confusing me
)
We take a card from A. Its first digit might be 1, with probability 5/30; or it might be 2, with probability 7/30; or it might be one of the other possibilities.
When they say "write down
P(A2) ", they're just asking you to write down the relevant probability - they're
NOT telling you that the first digit is 2.
Now the card is put in pack B (so B is larger) and we draw a card from B. Its first digit could be 1, with probability
B1, or one of the other probabilities.
So when they ask you about
P(A1∩B1) they're asking you to combine the probability from the 1st draw from A with the probability from the 1st draw from B.
You're not taking another card from the (smaller) pack A!
Does this help?