1. We can see that the total number of computers would be; 9 + 8 + 4 = <em>21 computers</em>
2. Of these 21 computers, 9 + 8 = 17 of them are not working, either with a virus or just not working
3. That leaves 4 out of the 21 computers working;<em> leaving you with a probability of 4/21 that Bill will choose a computer that works on the first try</em>!
