\(u_{\text {rel }}=u_1-u_2=0-(-5)=5 ms ^{-1}\)
\(t=3 \,s\)
\(a_{ rel }=a_1-a_2=-g-(-g)=0 \,ms ^{-2}\)
\(s_{ rel }=u_{ rel } t+\frac{1}{2} a_{ rel } t^2\)
\(\Rightarrow s_{ rel }=5 \times 3=15 \,m \quad\left(\because a_{ rel }=0\right)\)
So, \(s_{\text {rel }}=15 \,m\)
$OA, \,AB,\, BC,\, CD$