# A person standing on an escalator takes time $t_1$ to reach the top of a tower when the escalator is moving. He takes time $t_2$ to reach the top of the tower when the escalator is standing . How long will he take if he walks up a moving escalator ?
$\begin{array}{1 1} t_2 -t_1\\ t_1+t_2 \\ \large\frac{t_1t_2}{(t_1-t_2)} \\ \large\frac{t_1t_2}{(t_1+t_2)} \end{array}$