A building has 28 floors, and the number of stairs between every two floors is the same. It is known that A goes upstairs twice as fast as B. When A reaches the 13th floor, what floor is B on?
一座大楼共28层，每两层间楼梯阶数都相同．已知甲上楼的速度比乙快一倍，当甲到达第13层时，乙在第多少层．
<gadget id="calculator">13 - 1</gadget>
<output>12</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

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

<result>7</result>