There are 1640 peach trees and pear trees in the orchard. If the number of peach trees increases by 420 and the number of pear trees decreases by 420, the number of the two numbers will be the same. How many peach trees are there?
果园里有桃树和梨树共1640棵，若桃树增加420棵，梨树减少420棵，两种数的棵数就一样多，原来桃树多少棵．
<gadget id="calculator">420 * 2</gadget>
<output>840</output>

<gadget id="calculator">1_640 - 840</gadget>
<output>800</output>

<gadget id="calculator">800 / 2</gadget>
<output>400</output>

<result>400</result>