You can put this solution on YOUR website! I assume that 40' (or 40 feet) applies to the circumference of the circle since you di dnot point this out in your question.
Use C = 2pir
Let C = 40 and solve for r
Let pi = 3.14
40 = 2(3.14)r
40 = 6.28r
40/6.28 = r
6.4 = r
NOTE: I rounded 6.369 to the nearest tenths to get the radius answer of 6.4 for your circle.
Make sense?
