袋中有4枝笔和3枝蓝铅笔，如果闭着眼睛摸，一次必须摸出几枝铅笔才能保证至少有1枝蓝铅笔？
计算．=+===﹣=== 1=
口算．．
．
我会算．++ 1﹣﹣++﹣﹣++ 1﹣﹣．
1﹣=+=﹣=+=﹣=+=+=+=
=
+
=
=
=
﹣
=
= 1
=
．
．
+
+
1﹣
﹣
+
+
﹣
﹣
+
+
﹣
．
=
+
﹣
=
+
﹣
=
+
=
+
=
+
=
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