Imagine that 100000 atheletes tested so that 0.005(100000)= 500 use the steroid and 99500 don't. Of the 500 who use the steroid 0.95(500)= 465 test positive. Of the 99500 who don't use the steroid 0.01(99500)= 995 also test positive. That gives a total of 995+ 465= 1465 positive tests of whom 465 actually use steroids. The probability that a positive test means the athelete did use the steroid is 465/1465.