Large trucks can transport 7 tons of soil at a time, and small trucks can transport 4 tons of soil at a time. Now there are 70 large and small trucks, which can transport exactly 400 tons of soil at a time. May I ask: How many trucks are there?
大卡车一次能运7吨土，小卡车一次能运4吨土，现在有大、小卡车70辆，一次恰好能运土400吨．请问：大卡车有多少辆？
<gadget id="calculator">400 - 280</gadget>
<output>120</output>

<gadget id="calculator">120 / 3</gadget>
<output>40</output>

<result>40</result>