I think the easiest way to solve any problem like this would be to draw a Venn Diagram as I've done here.
File comment: Venn Diagram
Venn Diagram.JPG [ 17.94 KiB | Viewed 1413 times ]
Start by looking at the center most part of the diagram, the intersection of all three users. That's given to be 5.
The numbers mentioned for people using two machines also includes the number of people using all three. So to get the number of people using only two machines, subtract 5 from each of the numbers so you get 8,4 and 0 respectively as shown (intersection of two users)
So, now if you subtract the sum of number of two users and number of three users, you get the sole users of each machine, which goes into the other part of the circles.
Add all the numbers you have now and subtract it from 60, you get 11.
Mac + SPARC + SGI = 5
Mac + SPARC (Total) = 13
Mac + SPARC (Only) = 13 -5 = 8
Macs = 30
Mac Only = Macs - (Mac + SPARC) - (All three) = 30 - 8 - 5 = 17
Similarly, you get the other numbers as shown in the diagram. So your total number of users using these computers = 49
Hence nonusers = 60-49 =11.
Hope this helps.