What is the radius of a circle whose circumference is 16?

##8##
The circumference of a circle is equal to ##pi## which is a number ##~~3.14## multiplied by the diameter of the circle.
Therefore ##C=pid##.
We know that the circumference ##C## is ##16pi## so we can say that:
##16pi=pid##
We can divide both sides by ##pi## to see that ##16=d##.
We now know that the diameter of the circle is ##16##.
We also know that the diameter has twice the length of the radius.
In equation form: ##2r=d##
##2r=16##
##color(red)(r=8##
Note that since ##2r=d## the equation ##C=2pir## holds and can be used in place of ##C=pid##.