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