If the sales force is 100 staff then we can substitute for the percentages: 40 have laptops, 65 have desktops, and 24 have both.
Consider the following sets:
A those with laptops only
B those with desktops only
C those with both
A+C=40 represents those that have laptops;
B+C=65 represents those that have desktops;
C=24 represents those that have both, so A=40-24=16, B=65-24=41.
Those having either a laptop or a desktop=A+B=16+41=57.
Converting this to a percentage: 57% have either a laptop or a desktop (but not both).