There is a multiple value data type in Access 2007 that allows comma delimited data to be entered. Not sure this is the best way to go. I would create a new table called Answered that looks like this:
question_letter_id long integer
question_number long integer
answer_letter_id long integer
This way you can look up an letter with a question and see what letters had the answer. If you have a separate question table, you could store the questions separately and use this table to show what letters had which question.