For a woman who gives birth at age 35, the probability of having a baby suffering from Down syndrome is 1/270 [22]. A test of the amniotic fluid in the mother’s uterus is virtually 100 percent accurate in predicting Down syndrome but is expensive and can cause a miscarriage. A study of the effectiveness of an inexpensive blood test that does not risk miscarriage found that in 89 percent of the Down syndrome cases, the test gave a positive reading, while in 75 percent of the cases without Down syndrome, the test gave a negative reading. Of those cases where there is a positive reading, what fraction are false positives?