Prove that at least one of the real numbers $a_1 , a_2 , … , a_n$ is greater than or equal to the average of these numbers Prove that at least one of the real numbers $\,a_1 , a_2 , … , a_n$ is
greater than or equal to the average of these numbers. What kind of
proof did you use?
I think I should use contradiction but I don't know how should I use that.
Thank you so much.

Let $a =  \max\limits_{1 \leq i \leq n}a_i$
then you  can see that the average $ m = \frac{1}{n}\sum\limits_{i = 1}^n a_i  \leq \frac{a}{n}\sum\limits_{i = 1}^n 1 = a,  $ because each $a_i$  is less than or equal to a, hence the result.
