#### Answer

$\frac{3}{5}$

#### Work Step by Step

$3975=\underline5\times\underline5\times3\times\underline53$
$6625=\underline5\times\underline5\times5\times\underline53$
$GCF=5\times5\times53=1325$
$\frac{3975}{6625}=\frac{3975\div1325}{6625\div1325}=\frac{3}{5}$