It costs 900 yuan to buy 50 pens, how much will it cost to buy 28 pens?
买50支钢笔用了900元，买28支钢笔要用多少元．
<gadget id="calculator">900 / 50</gadget>
<output>18</output>

<gadget id="calculator">18 * 28</gadget>
<output>504</output>

<result>504</result>