Answer:
The number of feet of fencing that he needs will be equal to the perimeter of the circular patio.
We know that the perimeter of a circle of radius R is equal to:
P = 2*pi*R
where pi = 3.14
Then if the radius of the patio is 70ft, the perimeter of the patio will be:
P = 2*3.14*70ft = 439.6 ft
Then he needs 439.6 ft of fencing to enclose the patio.