If a garden has an area of 100 square feet how much fencing is needed to enclose the garden

Depends on the dimensions of the garden 
the most economical for resources is 10 ft*10 ft=100 sq ft

If 10 x 10 then fencing needed is 4 x 10 =40

if 5 x 20 then fencing needed is 5 x 2 + 20 x 2 = 50

so it depends on its shape, there are several answers if you stick with whole numbers and many more if you use fractions e.g 2.5 x 40