A standard screening test for Down's syndrome is based on a combination of maternal age and the level of serum alpha-feroprotein

A standard screening test for Down's syndrome is based on a combination of maternal age and the level of serum alpha-feroprotein. Using this test 80% of Down's syndrome cases can be identified at birth, while 5% of normals are detected as positive.
a. What is the sensitivity and specificity of the test?
b. Assume that 1/500 births have this genetic condition. What is the percentage of births who test positive using this test will actually have the condition?