I could be wrong but I got 28. I devided 420 by 15
Given the circle has a radius of 64, to get the value of the radius,r we shall proceed as follows.
Area=πr^2
Thus;
64=πr^2
64/π=r^2
r^2=20.3718
r=sqrt(20.3718)
r=4.5135
Therefore the length of radius,r=4.5135 units
Try 86.36%, I hope this helps!
