Could you please explain to me how to solve this problem?
In a city, $45\%$ of the population read magazine $A$, $55\%$ read magazine $B$ and $40\%$ read magazine $C$. $30\%$ read $A$ and $B$, $15\%$ read $A$ and $C$, $15\%$ read $B$ and $C$ and $10\%$ read all three magazines. How many percent read no magazine at all?
[The answer is apparently $20\%$.]