$$6 \cdot \frac{5}{12}:(\frac{2}{5}-0,4) = 6 \cdot \frac{5}{12}:(\frac{2}{5}-\frac{4}{10}) = 6 \cdot \frac{5}{12}:(\frac{2}{5}-\frac{2}{5}) = 6 \cdot \frac{5}{12}:0$$
Деление на ноль невозможно.