Let $\overrightarrow a $, $\overrightarrow b $ and $\overrightarrow c $ be three vectors such that $\left| {\overrightarrow a } \right| = \sqrt 3 $, $\left| {\overrightarrow b } \right| = 5,\overrightarrow b .\overrightarrow c = 10$ and the angle between $\overrightarrow b $ and $\overrightarrow c $ is ${\pi \over 3}$. If ${\overrightarrow a }$ is perpendicular to the vector $\overrightarrow b \times \overrightarrow c $ , then $\left| {\overrightarrow a \times \left( {\overrightarrow b \times \overrightarrow c } \right)} \right|$ is equal to _____.