#### Answer

The required speed of the balls is $100~m/s$
It seems unlikely that Dash can succeed because this speed is more than double the speed of a fastball thrown by a major league baseball pitcher.

#### Work Step by Step

The initial momentum of the system is zero. By conservation of momentum, if the balls gain a certain momentum $p_b$ in one direction, Dash and the skateboard will gain the same magnitude of momentum $p_s$ in the opposite direction.
We can find the required speed $v_b$ of the three balls:
$p_b = p_s$
$m_b~v_b = m_s~v_s$
$v_b = \frac{m_s~v_s}{m_b}$
$v_b = \frac{(60~kg)(0.50~m/s)}{(3)(0.10~kg)}$
$v_b = 100~m/s$
The required speed of the balls is $100~m/s$
It seems unlikely that Dash can succeed because this speed is more than double the speed of a fastball thrown by a major league baseball pitcher.