I managed to reach the correct (I think) answer without Google, the comments, or any knowledge of the Bayes' Theorem using the following method: 0.8% of women get positive mammographies and have breast cancer 9.696969696969697% of women get positive mammographies and don't have breast cancer 0.8 + 9.696969696969697 = 10.496969696969697 Therefore, 10.496969696969697% of women get positive mammographies As 0.8% of women get positive mammographies and have breast cancer, we simply need to calculate what percentage 0.8 is of 10.496969696969697 Therefore, there is a 7.621247113163973% chance that a woman with a positive mammography has breast cancer.