The speed at which mother goes upstairs is twice that of Xiaohua. They both start to climb the stairs from the first floor at the same time. When Xiaohua reaches the fourth floor, how many floors does mother reach?
妈妈上楼的速度是小华上楼速度的2倍，她们两人从一楼同时开始爬楼梯，当小华到达4楼时，妈妈到达多少楼．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">6 + 1</gadget>
<output>7</output>

<result>7</result>