The escalator in the shopping mall moves from bottom to top at a constant speed. Two children walk up and down the moving escalator. The girl walks from bottom to top, and the boy walks from top to bottom. As a result, the girl walks 30 steps to reach the upper floor, and the boy walks 90 steps. Arrive downstairs. If the number of stairs that boys walk per unit time is 3 times that of girls. Ask how many steps the escalator can see when the escalator is at rest.
商场自动扶梯以匀速由下往上行驶，两个孩子在行驶的扶梯上下走动，女孩由下往上走，男孩由上往下走，结果女孩走了30级到达楼上，男孩走了90级到达楼下．如果男孩单位时间内走的楼梯级数是女孩的3倍．问当时扶梯静止时，扶梯可看到的梯级共有多少级．
<gadget id="calculator">90 - 30</gadget>
<output>60</output>

<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">60 / 2</gadget>
<output>30</output>

<gadget id="calculator">30 + 30</gadget>
<output>60</output>

<result>60</result>