For a head on elastic collision

$${v_2} = {{m{u_1}} \over {m + 5m}} + {{m{u_1}} \over {m + 5m}}$$

$$ = {{2{u_1}} \over 6}$$ or $${{{u_1}} \over 3}$$

Initial kinetic energy of first mass $$ = {1 \over 2}mu_1^2$$

Final kinetic energy of second mass

$$ = {1 \over 2} \times 5m{\left( {{{{u_1}} \over 3}} \right)^2}$$

$$ = {5 \over 9}\left( {{1 \over 2}mu_1^2} \right)$$

$$\Rightarrow$$ kinetic energy transferred = 55% of initial kinetic energy of first colliding mass