\[7\frac{8}{21} - 4,375 = 7\frac{8}{21} - 4\frac{375}{1000} =\]
\[= 7\frac{8^{\backslash 8}}{21} - 4\frac{3^{\backslash 21}}{8} = 3\frac{64 - 63}{168} =\]
\[= 3\frac{1}{168}\]