The fourth grade planted 80 trees, 20 less than the fifth grade, and the sixth grade planted twice as many trees as the fourth grade. How many trees were planted in the three grades?
四年级植树80棵，比五年级少植20棵，六年级植树棵数是四年级的2倍，三个年级共植树多少棵？
<gadget id="calculator">80 * 2</gadget>
<output>160</output>

<gadget id="calculator">80 + 80 + 20 + 160</gadget>
<output>340</output>

<result>340</result>