Amountain bike wheel has a diameter of 26 in.
what is the approximate circumference of the wheel?

I shall assume that your mountain bike has circular wheels.

The circumference of any circle is (pi) x (the circle's diameter).

The circumference of the mountain bike wheel is  (pi) x (26 inches).

Since (pi) can never be completely written down, and no machine
can ever use the exact value of (pi), we need to decide on what
approximate value we will use for it.

A great many elementary school, middle school, and high school classes
use  3.14  for (pi).  This simple approximation is actually a pretty good one ...
it's only about 0.05% away from the actual value of (pi), so it will give us
quite an accurate answer.

      Circumference = (pi) x (26 inches)  = 

                           (3.14) x (26 inches)  =    81.64 inches .