Bài giải:

$\begin{align}& \left( 1-\frac{1}{4} \right)\times \left( 1-\frac{1}{5} \right)\times \left( 1-\frac{1}{6} \right)\times \left( 1-\frac{1}{7} \right) \\& =\left( \frac{4}{4}-\frac{1}{4} \right)\times \left( \frac{5}{5}-\frac{1}{5} \right)\times \left( \frac{6}{6}-\frac{1}{6} \right)\times \left( \frac{7}{7}-\frac{1}{7} \right) \\& =\frac{3}{4}\times \frac{4}{5}\times \frac{5}{6}\times \frac{6}{7} \\& =\frac{3}{7} \\\end{align}$