Ok...

See An Intuitive Explanation of Bayesian Reasoning By Eliezer Yudkowsky.

PS: I lied. "Almost 70%" is the wrong answer.

PSS: Only about 15% of doctors get this right. See:

• Casscells, Schoenberger, and Grayboys 1978;
• Eddy 1982;
• Gigerenzer and Hoffrage 1995;