Initially, both man and platform are stationary.

\(\therefore v _{\text {com }}= O\)

As there is no external force in horizontal direction, \(a _{ com }=0\)

\(\Rightarrow v _{ com }\) will remain same (zero) even after man starts moving.

After man starts moving,

Given: velocity of man w.r.t platform is \(v_{ m / p }=30 km / hr\).

Let recoil velocity of platform w.r.t ground be \(v_p\) and velocity of man w.r.t ground be \(v_m\).

\(\Rightarrow v_m=v_{m / p}+\left(-v_p\right)=30 km / hr -v_p\)

\(v_{\text {com }}=0 \text {, }\)

\(\Rightarrow \frac{m_m v_m+m_p\left(-v_p\right)}{m_m+m_p}=0\)

\(\Rightarrow100\left(30-v_p\right)-200 v_p=0\)

\(\Rightarrow v _{ p }=10\; km / hr\)

\(\therefore \text { Recoil speed of platform }=10\; km / hr\)