Cut four small squares with a side length of 3 cm from the four corners of a piece of iron sheet with a length of 26 cm, and then connect them to a rectangular iron box. The volume of this box is 840 cubic centimeters. How many centimeters is the original width of this iron sheet?
从一块长26厘米的铁皮的四角剪去四个边长是3厘米的小正方形，再捍接成长方体铁盒，这个盒子的容积是840立方厘米．这块铁皮原来宽是多少厘米？
<gadget id="calculator">26 - 3 - 3</gadget>
<output>20</output>

<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<gadget id="calculator">840 / 60</gadget>
<output>14</output>

<gadget id="calculator">14 + 3 + 3</gadget>
<output>20</output>

<result>20</result>