There are 400 pens and pencils in the stationery store. When the same number of pens and pencils are sold, there will be 50 pens and 250 pencils left. How many pens are there in the stationery store?
文具店有钢笔和铅笔共400支，当两种笔卖出同样多的支数时，钢笔还剩下50支，铅笔剩下250支．文具店原有钢笔多少支？
<gadget id="calculator">250 - 50</gadget>
<output>200</output>

<gadget id="calculator">400 - 200</gadget>
<output>200</output>

<gadget id="calculator">200 / 2</gadget>
<output>100</output>

<result>100</result>