The school organizes a car model competition. There are 28 participants in the third grade, which is twice the number of participants in the fourth grade, and the number of participants in the fifth grade is half of the total number of participants in the third and fourth grades. How many students from grades 3, 4, and 5 participated in the car model competition?
学校组织车模比赛．三年级有28人参加，是四年级参加人数的2倍，五年级参加的人数是三、四年级参加总人数的一半．三、四、五年级一共有多少人参加车模比赛？
<gadget id="calculator">28 / 2</gadget>
<output>14</output>

<gadget id="calculator">28 + 14</gadget>
<output>42</output>

<gadget id="calculator">42 / 2</gadget>
<output>21</output>

<gadget id="calculator">28 + 14 + 21</gadget>
<output>63</output>

<result>63</result>