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The smallest number is 21. Of the other three numbers if any one is greater than 21 then one of the numbers is less than 21 since the avearge is 21. For eg 19,21,23. If the largest is 21 then all the numbers have to be 21 for the same reason. So the answer is D.
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When the smallest (or largest) number equals the average, ALL numbers must be identical.

Given: Average of 7 numbers = 21

Statement 1: The smallest number is 21
Think about it: If the smallest number is already 21 (the average), can any number be greater than 21?
No! If even one number were above 21, it would pull the average UP — but there's nothing below 21 to balance it out.
Therefore: All 7 numbers must equal exactly 21.
When all numbers are identical, standard deviation = 0

SUFFICIENT

Statement 2: The greatest number is 21
Same logic in reverse: If the greatest number is already 21 (the average), can any number be less than 21?
No! If even one number were below 21, it would pull the average DOWN — but there's nothing above 21 to balance it out.
Therefore: All 7 numbers must equal exactly 21.
Standard deviation = 0

SUFFICIENT

Principle: If the minimum equals the average, or the maximum equals the average, then all values must be equal to the average. No variation is possible.

Answer: D

MathRevolution
If the average (arithmetic mean) of 7 numbers is 21, what is the standard deviation of the numbers?

1) The smallest number of them is 21
2) The greatest number of them is 21
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