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7個數,它們的平均數是6.9,則7個數之和是6.9×7=48.3 前四個數的平均數是29,前四個數的和是8.5×4=34 後四個數的平均數是10,後四個數的和是10×4=40 前四個數的和+後四個數的和=74=7個數之和+第4個數 囙此第4個數=74-48.3=25.7
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