A boy of mass m running horizontally with a velocity v jumps onto a...

Using the principle of conservation of linear momentum:

Initial momentum of the system = Final momentum of the system

The initial momentum is only due to the boy, as the skateboard is stationary:
$p_i = m v + (2m)(0) = m v$

After the jump, the boy and the skateboard move together with a common velocity $v_f$. The total mass is $m + 2m = 3m$.
Final momentum:
$p_f = (3m) v_f$

Equating initial and final momentum:
$m v = 3m v_f$

$v_f = \dfrac{v}{3}$

Answer: $\dfrac{v}{3}$