An advertising campaign is being developed to promote a new bookstore opening in the newest mall development. To develop an appropriate mailing list it has been decided to purchase lists of credit card holders from MasterCard and American Express. Combining the lists they find the following: 40% of the people on the list have only a MasterCard and 10% have only an American Express card. Another 20% hold both MasterCard and American Express. Finally, 30% of those on the list have neither card. Given that a person has a MasterCard is the probability that person also has an American Express Card?