Four numbers are 3, 5, 7 and one unknown number. Their mean is 5. What is the unknown number?

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A mean of 5 across 4 numbers requires a total of 20, and 3 + 5 + 7 = 15, so the unknown number is 20 - 15 = 5. Ruling out 5 because it matches the mean is a habit rather than a rule, since nothing stops a missing value from equalling the mean of the set it belongs to.