University Maths. - Stat. 7

Out of the 0.4% cancer patient, only 95% will show positive result. So the portion of contribution to positive results from actual patient is 0.4% * 95% = 0.38%
Out of the 99.6% non-patient, 5% will show positive results. Contribution to positive results from non- patient is 99.6% * 5% = 4.98%.
Given a test result is positive, the probability of the person having cancer = 0.38% / (0.38% + 4.98%) = 7.09%