The school organized tree planting activities. The first grade planted 21 trees, the second grade planted twice as many trees as the first grade, and the third grade planted twice as many trees as the second grade. How many trees were planted in the third grade?
学校组织植树活动，一年级植树21棵，二年级植的棵数是一年级的2倍，三年级植的棵数是二年级的2倍，三年级植树多少棵？
<gadget id="calculator">21 * 2 * 2</gadget>
<output>84</output>

<result>84</result>