A body $\mathrm{A}$ of mass $m=0.1 \mathrm{k}\mathrm{g}$ has an initial velocity of $3\hat{\mathrm{i}} \mathrm{m} {\mathrm{s}}^{-1}$. It collides elastically with another body $\mathrm{B}$ of the same mass which has an initial velocity of $5\hat{\mathrm{j}} \mathrm{m} {\mathrm{s}}^{-1}$. After the collision, $\mathrm{A}$ moves with a velocity $\overrightarrow{v}=4\left(\hat{\mathrm{i}}+\hat{\mathrm{j}}\right) \mathrm{m} {\mathrm{s}}^{-1}$. The energy of $\mathrm{B}$ after the collision is written as $\frac{x}{10} \mathrm{J}$. The value of $x$ is