A) 15
B) 18
C) 9
D) 12
Correct Answer: B
Solution :
[b] We know that for positive real numbers \[{{x}_{1}},\] \[{{x}_{2}},...,{{x}_{n}},\] We have \[\frac{\sum{x_{i}^{2}}}{n}\ge {{\left( \frac{\sum{{{x}_{i}}}}{n} \right)}^{2}}\Rightarrow \frac{400}{n}\ge {{\left( \frac{80}{n} \right)}^{2}}\] \[\Rightarrow n\ge 16.\] so only possible value of n=18
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