Ok. Maximum of C is 22, minimum is 18.

Sub those into the formula for circumference.

You get,

and

.

These will give you the maximum and minimum radii he could have had.

Solve for in both cases and sub that into the formula for volume. You'll get a maximum and minimum volume. Find which one is farthest from the volume obtained by setting C=20 and find out how far. That is the largest error.