There are two wooden strips with a length of 20 cm. They are fixed into a wooden strip with a length of 36 cm. They are fixed together in the middle. What is the length of the overlapping part?
有两根长都是20厘米的木条，定成一根长36厘米的木条，中间定在一起，重叠部分长多少厘米．
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">40 - 36</gadget>
<output>4</output>

<result>4</result>