The school organizes model airplane competitions. There are 28 participants in the third grade, twice as many participants in the fourth grade as in the third grade, and half of the total number of participants in the fifth grade. How many people in the fifth grade participate in the model airplane competition?
学校组织航模比赛．三年级有28人参加，四年级参加的人数是三年级的2倍，五年级参加的人数是三、四年级参加的总人数的一半．五年级有多少人参加航模比赛？
<gadget id="calculator">28 * 2</gadget>
<output>56</output>

<gadget id="calculator">56 + 28</gadget>
<output>84</output>

<gadget id="calculator">84 / 2</gadget>
<output>42</output>

<result>42</result>