The elder brother walks up the escalator from top to bottom and walks a total of 100 steps. The younger sister walks from the bottom to the end of the escalator and walks a total of 50 steps. If the number of steps the elder brother walks per unit time belongs to the younger sister 2 times, then when the escalator is stationary, how many steps can be seen.
哥哥沿着向上移动的自动扶梯从上向下走到底，共走了100级，妹妹沿着自动扶梯从底向上走到头，共走了50级，如果哥哥单位时间内走的级数是妹妹的2倍，那么当自动扶梯静止时，能看到的部分有多少级．
<gadget id="calculator">100 + 50</gadget>
<output>150</output>

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

<result>75</result>