area of circle

Question

A circular garden of radius 140m has a circular footpath 7m along its outer edge. Find the length of iron fencing needed to cover the outer edge of the footpath.

Answer 1

PLZ notice the "magic numbers" in this problem.  The radius and the footpath are multiples of 7, so using the estimate of 22/7 for Pi is expected.

 

The definition of Pi (∏) = (Circumference of circle)/(diameter of circle)

This means that the Circumference is ∏d or 2∏r.

 

When the garden and the footpath are considered together, the total radius is (140m+7m).  So,

     Circumference = 2*(22/7)*(147)

     Circumference = 924 m