Doudou and Dongdong have a stair climbing competition. When Doudou reaches the 4th floor, Dongdong runs to the 5th floor. According to this calculation, when Doudou reaches the 16th floor, which floor does Dongdong run to?
豆豆和东东两人爬楼梯比赛，豆豆跑到第4层时，东东跑到第5层，照这样计算，当豆豆跑到第16层时，东东跑到了第几层？
<gadget id="calculator">16 - 1</gadget>
<output>15</output>

<gadget id="calculator">15 / 3</gadget>
<output>5</output>

<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<result>21</result>