Two balls A and B are placed at the top of \( 180 \, \text{m} \) tall tower. Ball A is released from the top at \( t = 0 \, \text{s} \). Ball B is thrown vertically down 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 \( g = 10 \, \text{m s}^{-2} \) ] :
