There are three iron wires, the first one is 15 meters long, the second one is 13 meters long, and the third one is half the length of the first two iron wires. What is the total length of these three iron wires?
有三根铁丝，第一根长15米，第二根长13米，第三根的长度是前两根铁丝总和的一半，这三根铁丝一共长多少米．
<gadget id="calculator">13 + 15</gadget>
<output>28</output>

<gadget id="calculator">28 / 2</gadget>
<output>14</output>

<gadget id="calculator">13 + 15 + 14</gadget>
<output>42</output>

<result>42</result>