Xiaoli uses two identical triangles to assemble parallelograms to get three different parallelograms, whose perimeters are 18cm, 20cm, and 22cm. What is the perimeter of one of the triangles.
小丽用两个完全相同的三角形拼平行四边得到三种不同的平行四边形，其周长分别为18cm，20cm，22cm．其中一个三角形的周长为多少．
<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">22 / 2</gadget>
<output>11</output>

<gadget id="calculator">9 + 10 + 11</gadget>
<output>30</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<result>15</result>