A large cart and three small carts are used to transport 6 times a total of 120 tons of coal. It is known that the large cart needs to be transported twice before the small cart is transported. Find the weight of the small cart.
用一辆大车和三辆小车共同合运6次共运煤120吨，已知大车一次运的小车要2次才能运完，求小车载重量．
<gadget id="calculator">2 * 6</gadget>
<output>12</output>

<gadget id="calculator">3 * 6</gadget>
<output>18</output>

<gadget id="calculator">12 + 18</gadget>
<output>30</output>

<gadget id="calculator">120 / 30</gadget>
<output>4</output>

<result>4</result>