The escalator moves from bottom to top at a constant speed. A walks from the top down to the bottom, a total of 150 steps; B walks from the bottom up to the top, a total of 75 steps. If A's speed is three times that of B's, how many steps are there in the visible part of the escalator?
自动扶梯由下向上匀速运动，甲从顶部向下走到底部，共走了150级；乙从底部向上走到顶部，共走了75级．如果甲的速度是乙的速度的3倍，那么扶梯可见部分共有多少级？
<gadget id="calculator">150 * 3</gadget>
<output>450</output>

<gadget id="calculator">75 * 2</gadget>
<output>150</output>

<gadget id="calculator">450 + 150</gadget>
<output>600</output>

<gadget id="calculator">3 + 2</gadget>
<output>5</output>

<gadget id="calculator">600 / 5</gadget>
<output>120</output>

<result>120</result>