\[\boxed{\text{6.375.}}\]
\[Среднее\ арифметическое\ \]
\[равно:\ \]
\[(42,43 + 42,39 + 42,64 + 42,57)\ :4 =\]
\[= (84,82 + 85,21)\ :4 =\]
\[= 170,03\ :4 = 42,5075 \approx 42,51.\ \]
\[Ответ:42,51.\ \ \ \]