Use 9 cars and 18 horse-drawn carts to transport a batch of goods. The load of each car is equivalent to three times that of a horse-drawn cart. In this way, the car transports a total of 18 tons more than the horse-drawn cart. How many tons does each horse-drawn cart carry?
用9辆汽车和18辆马车运送一批货物，每辆汽车的载重量相当于一辆马车的3倍，这样汽车比马车共多运了18吨，求每辆马车运多少吨？
<gadget id="calculator">9 * 3</gadget>
<output>27</output>

<gadget id="calculator">27 - 18</gadget>
<output>9</output>

<gadget id="calculator">18 / 9</gadget>
<output>2</output>

<result>2</result>