10. Pay for an escalator moving upwards. A walks down from its top to its bottom, totaling 150 steps, B walks up from its bottom to its top, totaling 75 steps. Assuming A's speed (number of steps walked per unit time) is 3 times B's speed, how many steps of the escalator are visible at any given moment? (Assume this number is a constant)
10. Let $v_{1}, v_{2}, v$ represent the speeds (number of steps walked per unit time) of $A, B$, and the automatic escalator, respectively, and let $t_{1}, t_{2}, t$ represent the time taken by $A, B$, and the automatic escalator, respectively. From the problem, we have: $v_{1}=3 v_{2}, v_{1} t_{1}=150, v_{2} t_{2}=75$ $\rightarrow t_{2}: t_{1}=3: 2$. Let $x$ be the number of visible steps on the automatic escalator $\Rightarrow x=\left(v_{2}+v\right) t_{2}=\left(v_{1}-v\right) t_{1}$
$$
\begin{array}{l}
\Rightarrow x=75+v t_{2}=150-v t_{1} \Rightarrow t_{2}=\frac{x-75}{v}, \\
t_{1}=\frac{150-x}{v}>t_{2}: t_{1}=(x-75):(150-x) \\
=3: 2 \rightarrow 2(x-75)=3(150-x) \Rightarrow x=120 .
\end{array}
$$