I believe the answer would be 36%.
So, let's start with fractions. Giovanni is on the 16th floor out of 25; so, 16/25. Now, if we divide 16 by 25, it will give us a decimal, which is .64. Multiply this by 100, and we have a percent! So, the 16th floor is 64% up the building, if that makes sense.
After that, just subtract 64 from 100 to get the difference: 36%.
I really hope this helps!