Function cannot be used as p.m.f I have the following question:
Give one reason why the following function cannot be a probability mass function:
$$ p(x)= \frac{1}{10} (x+2)$$ where $x=1,2,3$
The solution that i have, but i'm not sure if its an answer, is that if I sum all possible outcomes, they are different than $1$. 

The sum is $12/10$ which is greater than $1$.
