The bearing limit of a spring is 15 kg. When the upper end is fixed and a 2 kg object is hung on the lower end, the measured length of the spring is 20 cm. If a 5 kg object is hung, the measured length of the spring is 23 cm. What is the original length of the spring in cm?
一根弹簧的承受极限是15千克，当上端固定，下端挂上2千克的物品时测得弹簧长20厘米，如果挂上5千克的物品，测得弹簧长23厘米，弹簧原来长多少厘米．
<gadget id="calculator">23 - 20</gadget>
<output>3</output>

<gadget id="calculator">5 - 2</gadget>
<output>3</output>

<gadget id="calculator">3 / 3</gadget>
<output>1</output>

<gadget id="calculator">5 * 1</gadget>
<output>5</output>

<gadget id="calculator">23 - 5</gadget>
<output>18</output>

<result>18</result>