Question: > 
> There is a group of finite aliens on a spaceship. Show that there are at least $2$ aliens who know the same number of aliens on the spaceship. 
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I was given a hint, and that was to use the pigeonhole principle. I think I can visually see it but I am unsure how to show it.

Answer: 
HINT: Let $n$ be the number of aliens on the ship. As noted in the comments, you must assume that $n\ge 2$. Suppose that A is an alien on the ship; how many of the others can it know? The largest possible answer is $n-1$, and the smallest is $0$. 

* How many possibilities is that?
* Is it actually possible for one of the aliens to know $n-1$ of the others and another to know $0$?
suppose there are 10 aliens for example .
alien 1 knows alien 2 , how many aliens does 2 know now ?? for the hypothesis to be false alien 2 already knows alien 1 and for him not to know just 1 lets assume he knows alien 3 thus knows 2 .
same for alien 3 , he must know 3 .. continueing onward when we get to alien 10 he knows 10 aliens but since he cant count himself thats a contradiction hence 2 aliens know same number of aliens .