A double-decker bus carries a total of 51 passengers. The number of passengers on the lower deck is twice that of the upper deck. How many passengers are there on the upper deck?
一辆双层巴士共载有乘客51人，下层的乘客是上层的2倍，上层有乘客多少人？
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">51 / 3</gadget>
<output>17</output>

<result>17</result>