Inner Radius of a pipe, given outer circumference, thickness

[WriterGirl]

What is the radius of the inside of the sewer pipe if the circumference of the outside of the pipe is 12.56 feet?

I'm puzzled. I attempted to discover the radius by plugging the circumference into the circumference formula and isolating R to find the radius, however, i came up with r=2, which seems incorrect for the pipe size...

 

[stapel]

What is the radius of the inside of the sewer pipe if the circumference of the outside of the pipe is 12.56 feet?

I'm puzzled. I attempted to discover the radius by....

In future, please show your work; this will make it much easier to locate any errors. Thank you! :wink:

The outer circumference is in terms of some outer radius R:

. . . . .\(\displaystyle 12.56\, =\, 2\pi R\)

Letting \(\displaystyle \pi\) equal 3.14 (because the value they gave for the circumference seems engineered for this approximation), we get R = 2.

The thickness of the pipe is the distance between the outer radius R and the inner radius r. Since "three inches" is "one-fourth foot", what then is the value of r? (Hint: It is not "2 feet"!)