If the circumference is 72 inches what is the diameter?

diameter = pi / circumference diameter = 72 / 3.1415926 = 22.918312 inches *

You've reversed the substitutions for the variables from your first line in the second line (pi/72 is 0.04365, not 22.918312). Although your value for the diameter is OK, your formula does not support it.

The formula for circumference is C=2*pi*r (read as "C=two pi R"), or more simply, C=pi*d. Therefore, if both sides are divided by pi (to get d on one side of the equation only) you get C/pi=d. Using the value given for C and if pi is rounded to 3.14159, the answer is d=72/3.14159; d=22.91833 I went to that length to show how to restate the standard equation. That ability is a fundamental skill in math, and I'd hazard to guess that the asker could have figured out the answer if someone had shown them how to do that. If not, then maybe it will help someone else in the future.