how to solve set theory
Answers
Set theory has its own notations and symbols that can seem unusual for many. In this tutorial, we look at some solved examples to understand how set theory works and the kind of problems it can be used to solve.
Definition
A set is a collection of objects.
It is usually represented in flower braces.
For example:
Set of natural numbers = {1,2,3,…..}
Set of whole numbers = {0,1,2,3,…..}
Each object is called an element of the set.
The set that contains all the elements of a given collection is called the universal set and is represented by the symbol ‘µ’, pronounced as ‘mu’.
For two sets A and B,
n(AᴜB) is the number of elements present in either of the sets A or B.
n(A∩B) is the number of elements present in both the sets A and B.
n(AᴜB) = n(A) + (n(B) – n(A∩B)
For three sets A, B and C,
n(AᴜBᴜC) = n(A) + n(B) + n(C) – n(A∩B) – n(B∩C) – n(C∩A) + n(A∩B∩C)
Consider the following example:
Question: In a class of 100 students, 35 like science and 45 like math. 10 like both. How many like either of them and how many like neither?
Solution:
Total number of students, n(µ) = 100
Number of science students, n(S) = 35
Number of math students, n(M) = 45
Number of students who like both, n(M∩S) = 10
Number of students who like either of them,
n(MᴜS) = n(M) + n(S) – n(M∩S)
→ 45+35-10 = 70
Number of students who like neither = n(µ) – n(MᴜS) = 100 – 70 = 30
The easiest way to solve problems on sets is by drawing Venn diagrams, as shown below.
Set Theory - Venn Diagram 1
As it is said, one picture is worth a thousand words. One Venn diagram can help solve the problem faster and save time. This is especially true when more than two categories are involved in the problem.
Let us see some more solved examples.
Problem 1: There are 30 students in a class. Among them, 8 students are learning both English and French. A total of 18 students are learning English. If every student is learning at least one language, how many students are learning French in total?
Solution:
The Venn diagram for this problem looks like this.
Set Theory - Venn Diagram 2
Every student is learning at least one language. Hence there is no one who fall in the category ‘neither’.
So in this case, n(EᴜF) = n(µ).
It is mentioned in the problem that a total of 18 are learning English. This DOES NOT mean that 18 are learning ONLY English. Only when the word ‘only’ is mentioned in the problem should we consider it so.
Now, 18 are learning English and 8 are learning both. This means that 18 – 8 = 10 are learning ONLY English.
n(µ) = 30, n(E) = 10
n(EᴜF) = n(E) + n(F) – n(E∩F)
30 = 18+ n(F) – 8
n(F) = 20
Therefore, total number of students learning French = 20.
Note: The question was only about the total number of students learning French and not about those learning ONLY French, which would have been a different answer, 12.
Finally, the Venn diagram looks like this.
Set Theory - Venn Diagram 3
Problem 2: Among a group of students, 50 played cricket, 50 played hockey and 40 played volley ball. 15 played both cricket and hockey, 20 played both hockey and volley ball, 15 played cricket and volley ball and 10 played all three. If every student played at least one game, find the number of students and how many played only cricket, only hockey and only volley ball?
Solution:
n(C) = 50, n(H) = 50, n(V) = 40
n(C∩H) = 15
n(H∩V) = 20
n(C∩V) = 15
n(C∩H∩V) = 10
No. of students who played at least one game
n(CᴜHᴜV) = n(C) + n(H) + n(V) – n(C∩H) – n(H∩V) – n(C∩V) + n(C∩H∩V)
= 50 + 50 + 40 – 15 – 20 – 15 + 10
Total number of students = 100.
Let a denote the number of people who played cricket and volleyball only.
Let b denote the number of people who played cricket and hockey only.
Let c denote the number of people who played hockey and volleyball only.
Let d denote the number of people who played all three games.
Accordingly, d = n (CnHnV) = 10
Now, n(CnV) = a + d = 15
n(CnH) = b + d = 15
n(HnV) = c + d = 20
Therefore, a = 15 – 10 = 5 [cricket and volleyball only]
b = 15 – 10 = 5 [cricket and hockey only]
c = 20 – 10 = 10 [hockey and volleyball only]
No. of students who played only cricket = n(C) – [a + b + d] = 50 – (5 + 5 + 10) = 30
No. of students who played only hockey = n(H) – [b + c + d] = 50 – ( 5 + 10 + 10) = 25
No. of students who played only volley ball = n(V) – [a + c + d] = 40 – (10 + 5 + 10) = 15
Set Theory solved example
Alternatively, we can solve it faster with the help of a Venn diagram.
The Venn diagram for the given information looks like this.
Set Theory - Venn Diagram 4
Subtracting the values in the intersections from the individual values gives us the number of students who played only one game.