Students from three classes in the sixth grade participated in the "Arbor Day" activity. The ratio of the trees planted by the first class to the other two classes is 1:2, the ratio of the trees planted by the second class to the other two classes is 2:7, and the third class planted 72 trees. How many trees did the three classes plant in total?
六年级三个班的同学参加"植树节"活动．一班与其余两个班植树棵树的比是1：2，二班与其余两个班植树棵树的比是2：7，三班植树72棵．三个班一共植树多少棵？
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">2 + 7</gadget>
<output>9</output>

<gadget id="calculator">2 / 9</gadget>
<output>2/9 = around 0.222222</output>

<gadget id="calculator">1 - (1/3) - (2/9)</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">72 / (4/9)</gadget>
<output>162</output>

<result>162</result>