Answer:
The total number of family members is 21.
Step-by-step explanation:
To solve this problem, we must build the Venn's Diagram of these sets.
I am going to say that:
-The set A represents those that would not go to a park.
-The set B represents those who would not go to a beach.
-The set C represents those who would not go to the family cottage.
The value d represents those who would go to all three places.
We have that:
![A = a + (A \cap B) + (A \cap C) + (A \cap B \cap C)](https://tex.z-dn.net/?f=A%20%3D%20a%20%2B%20%28A%20%5Ccap%20B%29%20%2B%20%28A%20%5Ccap%20C%29%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29)
In which a are those that would only not go to a park,
are those who would not got to a park or to the beach,
are those who would not go to a park or to the famili cottage. And
are those that would not go to any of these places.
By the same logic, we have:
![B = b + (B \cap C) + (A \cap B) + (A \cap B \cap C)](https://tex.z-dn.net/?f=B%20%3D%20b%20%2B%20%28B%20%5Ccap%20C%29%20%2B%20%28A%20%5Ccap%20B%29%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29)
![C = c + (A \cap C) + (B \cap C) + (A \cap B \cap C)](https://tex.z-dn.net/?f=C%20%3D%20c%20%2B%20%28A%20%5Ccap%20C%29%20%2B%20%28B%20%5Ccap%20C%29%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29)
This diagram has the following values:
![a,b,c,d,(A \cap B), (A \cap C), (B \cap C), (A \cap B \cap C)](https://tex.z-dn.net/?f=a%2Cb%2Cc%2Cd%2C%28A%20%5Ccap%20B%29%2C%20%28A%20%5Ccap%20C%29%2C%20%28B%20%5Ccap%20C%29%2C%20%28A%20%5Ccap%20B%20%5Ccap%20C%29)
The total number of family members is the sum of all these values:
![T = a + b + c + d + (A \cap B) + (A \cap C) + (B \cap C) + (A \cap B \cap C)](https://tex.z-dn.net/?f=T%20%3D%20a%20%2B%20b%20%2B%20c%20%2B%20d%20%2B%20%28A%20%5Ccap%20B%29%20%2B%20%28A%20%5Ccap%20C%29%20%2B%20%28B%20%5Ccap%20C%29%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29)
We start finding the values from the intersection of the three sets
5 would not go to a park or a beach or to the family cottage.
This means that ![A \cap B \cap C = 5](https://tex.z-dn.net/?f=A%20%5Ccap%20B%20%5Ccap%20C%20%3D%205)
1 would go to all three places. This means that
.
8 would go to neither a park nor the family cottage
This means that:
![A \cap C + (A \cap B \cap C) = 8](https://tex.z-dn.net/?f=A%20%5Ccap%20C%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29%20%3D%208)
![A \cap C = 3](https://tex.z-dn.net/?f=A%20%5Ccap%20C%20%3D%203)
8 would go to neither a beach nor the family cottage
![B \cap C + (A \cap B \cap C) = 8](https://tex.z-dn.net/?f=B%20%5Ccap%20C%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29%20%3D%208)
![B \cap C = 3](https://tex.z-dn.net/?f=B%20%5Ccap%20C%20%3D%203)
7 would go to neither a park nor a beach
![A \cap B + (A \cap B \cap C) = 7](https://tex.z-dn.net/?f=A%20%5Ccap%20B%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29%20%3D%207)
![A \cap B = 2](https://tex.z-dn.net/?f=A%20%5Ccap%20B%20%3D%202)
15 would not go to the family cottage
![C = 15](https://tex.z-dn.net/?f=C%20%3D%2015)
![C = c + (A \cap C) + (B \cap C) + (A \cap B \cap C)](https://tex.z-dn.net/?f=C%20%3D%20c%20%2B%20%28A%20%5Ccap%20C%29%20%2B%20%28B%20%5Ccap%20C%29%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29)
![15 = c + 3 + 3 + 5](https://tex.z-dn.net/?f=15%20%3D%20c%20%2B%203%20%2B%203%20%2B%205)
![c = 4](https://tex.z-dn.net/?f=c%20%3D%204)
12 would not go to a beach
![B = 12](https://tex.z-dn.net/?f=B%20%3D%2012)
![B = b + (B \cap C) + (A \cap B) + (A \cap B \cap C)](https://tex.z-dn.net/?f=B%20%3D%20b%20%2B%20%28B%20%5Ccap%20C%29%20%2B%20%28A%20%5Ccap%20B%29%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29)
![12 = b + 3 + 2 + 5](https://tex.z-dn.net/?f=12%20%3D%20b%20%2B%203%20%2B%202%20%2B%205)
![b = 2](https://tex.z-dn.net/?f=b%20%3D%202)
11 would not go to a park
![A = 11](https://tex.z-dn.net/?f=A%20%3D%2011)
![A = a + (A \cap B) + (A \cap C) + (A \cap B \cap C)](https://tex.z-dn.net/?f=A%20%3D%20a%20%2B%20%28A%20%5Ccap%20B%29%20%2B%20%28A%20%5Ccap%20C%29%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29)
![11 = a + 2 + 3 + 5](https://tex.z-dn.net/?f=11%20%3D%20a%20%2B%202%20%2B%203%20%2B%205)
![a = 1](https://tex.z-dn.net/?f=a%20%3D%201)
Now, we can find the total number of family members.
![T = a + b + c + d + (A \cap B) + (A \cap C) + (B \cap C) + (A \cap B \cap C)](https://tex.z-dn.net/?f=T%20%3D%20a%20%2B%20b%20%2B%20c%20%2B%20d%20%2B%20%28A%20%5Ccap%20B%29%20%2B%20%28A%20%5Ccap%20C%29%20%2B%20%28B%20%5Ccap%20C%29%20%2B%20%28A%20%5Ccap%20B%20%5Ccap%20C%29)
![T = 1 + 2 + 4 + 1 + 2 + 3 + 3 + 5](https://tex.z-dn.net/?f=T%20%3D%201%20%2B%202%20%2B%204%20%2B%201%20%2B%202%20%2B%203%20%2B%203%20%2B%205)
![T = 21](https://tex.z-dn.net/?f=T%20%3D%2021)
The total number of family members is 21.