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In triangle $\text{ABC}$, angle $\text{B}$ is $90$ degrees and side $\text{AC}$ has a length of $20$.

If the $\text{cos A}=\cfrac{2}{5}$, then what is the length of side $\text{BC}$?

$8$

$12$

$16$

${4\sqrt{21}}$

${12\sqrt{2}}$