Answer:
91 people take Russian
26 people take French and Russian but not German
Step-by-step explanation:
To solve this problem, we must build the Venn's Diagram of this set.
I am going to say that:
-The set A represents the students that take French.
-The set B represents the students that take German
-The set C represents the students that take Russian.
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 is the number of students that take only Franch, A \cap B is the number of students that take both French and German , A \cap C is the number of students that take both French and Russian and A \cap B \cap C is the number of students that take French, German and Russian.
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 subsets:
![a,b,c,(A \cap B), (A \cap C), (B \cap C), (A \cap B \cap C)](https://tex.z-dn.net/?f=a%2Cb%2Cc%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)
There are 155 people in my school. This means that:
![a + b + c + (A \cap B) + (A \cap C) + (B \cap C) + (A \cap B \cap C) = 155](https://tex.z-dn.net/?f=a%20%2B%20b%20%2B%20c%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%20%3D%20155)
The problem states that:
90 take Franch, so:
![A = 90](https://tex.z-dn.net/?f=A%20%3D%2090)
83 take German, so:
![B = 83](https://tex.z-dn.net/?f=B%20%3D%2083)
22 take French, Russian, and German, so:
![A \cap B \cap C = 22](https://tex.z-dn.net/?f=A%20%5Ccap%20B%20%5Ccap%20C%20%3D%2022)
42 take French and German, so:
![A \cap B = 42 - (A \cap B \cap C) = 42 - 22 = 20](https://tex.z-dn.net/?f=A%20%5Ccap%20B%20%3D%2042%20-%20%28A%20%5Ccap%20B%20%5Ccap%20C%29%20%3D%2042%20-%2022%20%3D%2020)
41 take German and Russian, so:
![B \cap C = 41 - (A \cap B \cap C) = 41 - 22 = 19](https://tex.z-dn.net/?f=B%20%5Ccap%20C%20%3D%2041%20-%20%28A%20%5Ccap%20B%20%5Ccap%20C%29%20%3D%2041%20-%2022%20%3D%2019)
22 take French as their only foreign language, so:
![a = 22](https://tex.z-dn.net/?f=a%20%3D%2022)
Solution:
(1) How many take Russian?
![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)
![C = c + (A \cap C) + 19 + 22](https://tex.z-dn.net/?f=C%20%3D%20c%20%2B%20%28A%20%5Ccap%20C%29%20%2B%2019%20%2B%2022)
![C = c + (A \cap C) + 41](https://tex.z-dn.net/?f=C%20%3D%20c%20%2B%20%28A%20%5Ccap%20C%29%20%2B%2041)
First we need to find
, that is the number of students that take French and Russian but not German. For this, we have to go to the following equation:
![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)
![90 = 22 + 20 + (A \cap C) + 22](https://tex.z-dn.net/?f=90%20%3D%2022%20%2B%2020%20%2B%20%28A%20%5Ccap%20C%29%20%2B%2022)
.
![(A \cap C) = 26](https://tex.z-dn.net/?f=%28A%20%5Ccap%20C%29%20%3D%2026)
----------------------------
The number of students that take Russian is:
![C = c + 26 + 41](https://tex.z-dn.net/?f=C%20%3D%20c%20%2B%2026%20%2B%2041)
![C = c + 67](https://tex.z-dn.net/?f=C%20%3D%20c%20%2B%2067)
------------------------------
Now we have to find c, that we can find in the equation that sums all the subsets:
![a + b + c + (A \cap B) + (A \cap C) + (B \cap C) + (A \cap B \cap C) = 155](https://tex.z-dn.net/?f=a%20%2B%20b%20%2B%20c%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%20%3D%20155)
![22 + b + c + 20 + 26 + 19 + 22 = 155](https://tex.z-dn.net/?f=22%20%2B%20b%20%2B%20c%20%2B%2020%20%2B%2026%20%2B%2019%20%2B%2022%20%3D%20155)
![b + c + 109= 155](https://tex.z-dn.net/?f=b%20%2B%20c%20%2B%20109%3D%20155)
![b + c = 46](https://tex.z-dn.net/?f=b%20%2B%20c%20%3D%2046)
For this, we have to find b, that is the number of students that take only German. Then we go to this eqaution:
![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)
![B = b + 19 + 20 + 22](https://tex.z-dn.net/?f=B%20%3D%20b%20%2B%2019%20%2B%2020%20%2B%2022)
![B = b + 61](https://tex.z-dn.net/?f=B%20%3D%20b%20%2B%2061)
![b + 61 = 83](https://tex.z-dn.net/?f=b%20%2B%2061%20%3D%2083)
![b = 22](https://tex.z-dn.net/?f=b%20%3D%2022)
-------
![b + c = 46](https://tex.z-dn.net/?f=b%20%2B%20c%20%3D%2046)
![c = 46 - b](https://tex.z-dn.net/?f=c%20%3D%2046%20-%20b)
![c = 24](https://tex.z-dn.net/?f=c%20%3D%2024)
The number of people that take Russian is:
![C = c + 67](https://tex.z-dn.net/?f=C%20%3D%20c%20%2B%2067)
![C = 24 + 67](https://tex.z-dn.net/?f=C%20%3D%2024%20%2B%2067)
![C = 91](https://tex.z-dn.net/?f=C%20%3D%2091)
91 people take Russian
(2) How many take French and Russian but not German?
![(A \cap C) = 26](https://tex.z-dn.net/?f=%28A%20%5Ccap%20C%29%20%3D%2026)
26 people take French and Russian but not German