Number of yards in a mile = 1760, number of feet in a yard = 3, no. of feet in a mile=3×1760=5280.
In a square mile we have 5280×5280=27,878,400 sq ft. So 1 sq ft=1/27878400 sq mile.
In 100 sq ft we have 100/27878400=1/278784 sq mile=0.000003587 sq mile approx.