There is a seven-story pagoda, the number of lamps lit on each floor is equal to twice that of the upper floor, and a total of 381 lamps are lit. Find how many lamps are lit on the top floor.
有一个七层塔，每一层所点灯的盏数都等于上一层的2倍，一共点了381盏灯．求顶层点了多少盏灯．
<gadget id="calculator">1 + 2 + 4 + 8 + 16 + 32 + 64</gadget>
<output>127</output>

<gadget id="calculator">381 / 127</gadget>
<output>3</output>

<result>3</result>