Weigh an object with a spring scale. If the object weighs 2 kg, the length of the spring is 12.5 cm; if the object weighs 6 kg, the length of the spring is 13.5 cm. What is the length of the spring in centimeters when the object is not weighed?
用弹簧秤称物体，称2千克的物体，弹簧长12.5厘米；称6千克的物体，弹簧长13.5厘米．没有称物体时，弹簧长多少厘米．
<gadget id="calculator">13.5 - 12.5</gadget>
<output>1</output>

<gadget id="calculator">6 - 2</gadget>
<output>4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) * 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">12.5 - (1/2)</gadget>
<output>12</output>

<result>12</result>