As the figure shows, ball A is dropped from the top of a tower $100\ \text{m}$ high. At the same moment, ball B is thrown straight up from the foot of the tower at $25\ \text{m/s}$. When do they meet? Ignore the air.

Fig. 1: A tower 100 m high. Ball A is at its top, about to be dropped. Ball B is on the ground near its foot, thrown straight up at 25 m/s.