Soldiers of the People's Liberation Army volunteer to plant trees. A total of 840 trees are to be planted. 153 trees have been planted. How many more trees need to be planted?
解放军战士义务植树，一共要植840棵，已经植树153棵，还需再植树多少棵？
<gadget id="calculator">840 - 153</gadget>
<output>687</output>

<result>687</result>