The problem is:

> 
> In some country, there are 3 newspapers. 20% of the population read
>  newspaper A, 16% read B, and 14% read C. 8% of the population read
>  both A and B, 5% read A and C, and 4% read B and C. 4% read A, B
>  and C.
> 
> 
> 

How to find the probability that someone read exactly 1 newspaper?

Currently i have:

$P((A\setminus (B\cup C))\cup(B\setminus (A\cup C))\cup(C\setminus (A\cup B))
=
P(A\setminus (B\cup C))+P(B\setminus (A\cup C))+P(C\setminus (A\cup B))
=P(A)-P(B\cup C)+P(B)-P(A\cup C)+P(C)-P(A\cup B)$

Where $P(B\cup C)=P(B)+P(C)-P(B\cap C)$ and similar for the others.

However this approach is leading me to a negative value.

What's the problem here?