Just in case a picture helps...

... with W = well, I = ill, P = testing positive, N = testing negative...

See here Conditional probability - Wikipedia, the free encyclopedia for a more formal treatment of a very similar problem (see under the heading 'An example'), but from the tree you can see that

(b) is (398 + 99) / 20,000

and (c) is 99 / (398 + 99)

(a) is disconcertingly more obvious and (if I'm not mis-reading it) just 98%. Hope this helps.

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