Mean balance

Given information about the mean, can you work out the missing numbers?
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The mean of the set of numbers 8, 4, 5, 4, 
Image
Mean balance
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 and 6 is 6.

If another 
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Mean balance
 is added to the set, then the mean is still 6.

What numbers do 
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 and 
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 represent?

This problem is adapted from the World Mathematics Championships