If 24 inches is the diameter of the circle then multiply by 3.14 to find the circumference. 3.14 is known a Pi (pie) and is a constant that works with any size circle to find the circumference when the diameter is known
24x3.14 = 75.36
But if you do the sum on your calculator and use the Pi symbol x 24 you get 75.3982 and that is because the decimal fraction part of 3.1415926---------- And goes on forever. Same as the fraction 1/3 expressed as a decimal fraction is 0.3333333333333333 recurring forever
But in this case it is practical for everyday purposes to use 3.14