There are 840 apple trees, pear trees, and peach trees in the orchard. The number of pear trees is twice that of peach trees, and the number of apple trees is three times that of peach trees. How many apple trees are there?
果园里有苹果树、梨树、桃树共840棵，梨树棵数是桃树的2倍，苹果树是桃树的3倍，苹果树有多少棵．
<gadget id="calculator">2 + 3 + 1</gadget>
<output>6</output>

<gadget id="calculator">840 / 6</gadget>
<output>140</output>

<gadget id="calculator">140 * 3</gadget>
<output>420</output>

<result>420</result>