There are 36 students rowing in the lake, there are 7 boats in total, and each boat is just full. Among them, each big boat takes 6 people, and each small boat takes 4 people. How many big ships are there?
湖中有36名同学在划船，一共有7只船，而且每只船正好坐满．其中每只大船坐6人，每只小船坐4人．大船有几只？
<gadget id="calculator">7 * 6</gadget>
<output>42</output>

<gadget id="calculator">42 - 36</gadget>
<output>6</output>

<gadget id="calculator">6 - 4</gadget>
<output>2</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">7 - 3</gadget>
<output>4</output>

<result>4</result>