Two balls $A$ and $B$ are placed at the top of 180 m tall tower. Ball $A$ is released from the top at $t=0 \mathrm{~s}$. Ball $B$ is thrown vertically down with an initial velocity ' $u$ ' at $t=2 \mathrm{~s}$. After a certain time, both balls meet 100 m above the ground. Find the value of ' u ' in $\mathrm{ms}^{-1}$. [use $\mathrm{g}=10 \mathrm{~ms}^{-2}$ ] :