Answer
The speed of the raft is $0.50~m/s$ in the opposite direction from the person's motion.
Work Step by Step
We can assume the initial momentum of the person and the raft is zero. By conservation of momentum, if the person gains a certain momentum $p_p$ in one direction, the raft will gain the same magnitude of momentum $p_r$ in the opposite direction.
We can find the speed $v_r$ of the raft:
$p_r = p_p$
$m_r~v_r = m_p~v_p$
$v_r = \frac{m_p~v_p}{m_r}$
$v_r = \frac{(55~kg)(0.91~m/s)}{100.0~kg}$
$v_r = 0.50~m/s$
The speed of the raft is $0.50~m/s$ in the opposite direction from the person's motion.
