Xiaoming watered 3 rows of trees, and Xiaohong watered 3 rows of trees. The two watered a total of 72 trees. How many trees are there on average in each row?
小明浇了3行树，小红浇了3行树，两人一共浇了72棵树，平均每行有几棵？
<gadget id="calculator">3 + 3</gadget>
<output>6</output>

<gadget id="calculator">72 / 6</gadget>
<output>12</output>

<result>12</result>