Question

A student calculates that the circumference of a tire with a rim diameter of 15 inches and a sidewall width of 5.5 inches is 64.40 inches by doing the following calculation: (15+5.5)xpi. Explain the error.

Answer 1

we know that 
the formula of circumference is
C=pi*D
diameter=15 in
so
C=15*pi-----> 47.10  in

the error was having added to the diameter the sidewall width for the calculation of the circumference