Answer
8 bags are needed.
Work Step by Step
$area=180\ ft\times175\ ft=31500\ sqft$
$\frac{1\ bag}{4000\ sqft}=\frac{x\ bag}{31500 \ sqft}$
Use cross multiplication to solve for x.
$4000x=1\times31500$
$4000x\div4000=31500\div4000$
$x=7.875$