If I did this correctly all you hade to do is work in reverse.
1.) Divide 16 by pi (3.14) To get the diameter.16/ 3.14 =5.095541401 or 5.1
2.) Since Diameter is just the radius doubled divide 5.1 by 2. 5.1 /2 = 2.55
3.) Plug it in, use the 2pir equation to see if it works. 2X3.14X2.55= 16.014 or 16.
I hope this helps, let me know if you need further explanation.