解题思路：
(1)设三年级有
x
名志愿者，由题意可列得方程
x
=
(
18
+
30
+
x
)
×
20
%
，求解此方程即可得到结果，二年级所占的百分比为
1
−
50
%
−
20
%
=
30
%
，然后根据这些数据将两幅统计图补充完整即可；
(2)首先根据题意画出树状图，然后由树状图可以看出，有
12
种等可能的结果，其中两人都是二年级志愿者的情况有两种，从而求出两名队长都是二年级志愿者的概率。
试题解析：(1)设三年级有
x
名志愿者，由题意得
x
=
"
(
18
+
30
+
x
)
×
20
%
"
.
解得
x
=
12.
答：三年级有
12
名志愿者
.
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1
分
如图所示：
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3
分
(2)用
A
表示一年级队长候选人，B.
C
表示二年级队长候选人，
D
表示三年级队长候选人，树形图为
⋅
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5
分
从树形图可以看出，有
12
种等可能的结果，其中两人都是二年级志愿者的情况有两种，
所以
P
(
两名队长都是二年级志愿者
)
=
.
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7
分
（1）三年级有12名志愿者，两幅统计图补充完整见解析；
（2）两名队长都是二年级志愿者的概率为
.
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