In the shopping mall, Xiao Ming descends 120 steps from the top of the moving escalator to the bottom, and then climbs 90 steps from the bottom to the top. The number of steps from the bottom to the top of the automatic staircase is constant, assuming that the number of steps that Xiaoming descends per unit time is twice the number of steps he ascends. What is the number of steps from the bottom to the top of the escalator?
在商场里，小明从正在向上移动的自动楼梯顶部下120级台阶到达底部，然后从底部上90级台阶回到顶部．自动楼梯从底部到顶部的台阶数是不变的，假设小明单位时间内下台阶数是他上台阶数的2倍．则该自动楼梯从底到顶的台阶数为多少．
<result>108</result>