There are 15 floors in a building, and each floor is the same height. A goes upstairs twice as fast as B. When B reaches the 6th floor, what floor is A on?
一座大楼共15层，每层一样高，甲上楼的速度比乙快一倍，当乙到达第6层时，甲在第多少层．
<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">10 + 1</gadget>
<output>11</output>

<result>11</result>