Well say you have 10% and you want to change it to a decimal. 10% is the same as 10/100 or 10 divided by 100 which makes us move the decimal to the left twice.
I hope I helped :)
The circumference must be within half the difference of the average of those two numbers:
... |c -28.75| ≤ 0.25 . . . . where c is the circumference in inches