# How to get the number of target audience?

I am currently working on a project, a sample start-up. Suppose I have these data:

$$N$$ = total population.

$$n_1$$ = number of mall shoppers.

$$n_2$$ = number of people aged from 18-65.

$$n_3$$ = number of people with smartphones.

The question is how do I get the number of mall shoppers who are aged 18-65 and also have smartphones from the total population of the country? This number would then result to the approximate target audience for my start-up project but I don't know how to get it.

Let $$A$$, $$B$$, and $$C$$ be the sets of people in your population, such that $$\vert A \cup B \cup C \vert = N$$. If you treat these as proportions (i.e. A is the proportion of people in the population which shop at a mall) then you perhaps can use some laws of probability to find what you need.
You want to know $$P(A \cap B \cap C)$$ given that $$P(A) = n_1/N$$, $$P(B) = n_2/N$$, and $$P(C) = n_3/N$$. In order to do this, you will first need to know the size of the pairwise intersections (e.g. you need to know how Many people shop at a mall AND are between 18-65). Do you have that information?