Motion in a Straight Line: JEE Mains 2022 June 29 Shift 1

Two balls $A$ and $B$ are placed at the top of a $180\ \text{m}$ tall tower. Ball $A$ is released from the top at $t=0\ \text{s}$. Ball $B$ is thrown vertically downward with an initial velocity $u$ at $t=2\ \text{s}$. After a certain time, both balls meet $100\ \text{m}$ above the ground. Find the value of $u$ in $\text{m s}^{-1}$. [use $g=10\ \text{m s}^{-2}$]