here is my hastily drawn MSPain Venn Diagram
I've filled in all the information we're given.
Assuming 100 employees...
75 have a desktop AND a laptop
60 have a desktop AND a PDA
but then they say 95 employees have a desktop.
We have 135 people already listed for D though. This means some of them have to be moved from a 2-item category to the all-3 category. By moving people into the center we get to count them for both lists (towards the 75 and the 60) while they still only take up 1 spot on the 95.
75+60 = 135 - 95 = 40
This means that 40 people must be counted in the middle for it to work.
We can take that 40 from the 75, the 60 or 20 from each. It doesn't matter, we've just found our answer that a minimum of 40% have all 3.