Logical Reasoning • Topic 2 of 3

Venn Diagrams and Critical Thinking

What are Venn Diagrams?

Venn diagrams are visual representations that show relationships between different sets of items. Circles (or other shapes) overlap to show common elements.

Basic Venn Diagram Components:

ComponentSymbolMeaning
Universal SetRectangleAll possible elements
Set ACircleElements belonging to group A
Set BCircleElements belonging to group B
IntersectionOverlap regionElements in BOTH A and B
UnionAll circles combinedElements in A OR B (or both)
ComplementOutside circlesElements NOT in the set

Venn Diagram Regions (Two Sets):

`

┌─────────────────────────────────────────────┐

│ Universal Set │

│ ┌─────────────┐ ┌─────────────┐ │

│ │ A │ │ B │ │

│ │ ┌─────┐ │ │ │ │

│ │ │A∩B │ │ │ │ │

│ │ └─────┘ │ │ │ │

│ └─────────────┘ └─────────────┘ │

│ │

│ Outside: Elements in neither A nor B │

└─────────────────────────────────────────────┘

`

Set Notation:

SymbolMeaningExample
A ∩ BA intersection B (in both){2, 4}
A ∪ BA union B (in A or B or both){1,2,3,4,5}
A'Complement of A (not in A){5,6}
n(A)Number of elements in An(A)=3

What is Critical Thinking?

Critical thinking is the ability to analyze information objectively, evaluate arguments, and make logical decisions. It involves questioning assumptions and identifying patterns.

Critical Thinking Skills:

  1. Observation: Noticing details
  2. Analysis: Breaking down information
  3. Inference: Drawing conclusions from evidence
  4. Evaluation: Assessing strengths and weaknesses
  5. Problem-solving: Finding solutions logically

Real-life Applications:

  • Solving mystery puzzles
  • Analyzing survey data
  • Making informed decisions
  • Evaluating advertisements
  • Understanding probability
Venn Diagrams and Set TheoryStudents who like Cricket (C) and Football (F)Cricketonly18Both7Footballonly12n(C) = 25, n(F) = 19Set Formulasn(C ∪ F) = n(C) + n(F) − n(C ∩ F)= 25 + 19 − 7 = 37n(C only) = n(C) − n(C ∩ F)= 25 − 7 = 18n(F only) = n(F) − n(C ∩ F)= 19 − 7 = 12∪ = Union (OR), ∩ = Intersection (AND)A' = Complement (NOT A)
1
Worked Example

In a class of 40 students, 25 like mathematics, 20 like science, and 10 like both. How many students like neither subject?

Solution
  • Step 1: Draw Venn diagram with two circles (Math, Science)
  • Step 2: n(Math) = 25, n(Science) = 20, n(Math ∩ Science) = 10
  • Step 3: Only Math = 25 - 10 = 15
  • Step 4: Only Science = 20 - 10 = 10
  • Step 5: Total liking at least one = 15 + 10 + 10 = 35
  • Step 6: Students liking neither = Total - 35 = 40 - 35 = 5

Answer: 5 students like neither subject.

2
Worked Example

In a survey of 100 people, 60 read newspaper A, 45 read newspaper B, and 25 read both. How many read at least one newspaper?

Solution
  • Step 1: n(A) = 60, n(B) = 45, n(A ∩ B) = 25
  • Step 2: Use formula: n(A ∪ B) = n(A) + n(B) - n(A ∩ B)
  • Step 3: n(A ∪ B) = 60 + 45 - 25 = 105 - 25 = 80
  • Step 4: So 80 people read at least one newspaper

Answer: 80 people read at least one newspaper.

3
Worked Example

**Example 3 (Critical Thinking):** Five friends - Alice, Bob, Carol, David, and Emma - are sitting in a row. Alice is not at either end. Bob is next to Carol. David is between Emma and Alice. Who is sitting in the middle? *Solution:* - Step 1: Represent positions as 1, 2, 3, 4, 5 (left to right) -

Solution
  • Step 1: Represent positions as 1, 2, 3, 4, 5 (left to right)
  • Step 2: Alice is not at ends → Alice in position 2, 3, or 4
  • Step 3: Bob is next to Carol → BC or CB (adjacent)
  • Step 4: David is between Emma and Alice → E-D-A or A-D-E
  • Step 5: Try Alice in position 3 (middle):
  • If Alice in 3, David in 2 or 4 (must be between Emma and Alice)
  • If David in 2, then Emma in 1 (E-D-A) → positions: 1=Emma,2=David,3=Alice
  • Remaining positions 4 and 5 for Bob and Carol (adjacent) → works
  • So arrangement: Emma, David, Alice, Bob, Carol (or Carol, Bob)
  • Step 6: Alice is in position 3 (middle)

Answer: Alice is sitting in the middle.

Key Points

  • Venn diagrams show relationships between sets visually
  • Intersection (∩): elements in BOTH sets
  • Union (∪): elements in EITHER set (or both)
  • Formula: n(A ∪ B) = n(A) + n(B) - n(A ∩ B)
  • Start filling Venn diagram from the innermost intersection outward
  • Critical thinking involves analyzing, evaluating, and inferring logically
  • Use elimination and systematic testing to solve reasoning puzzles
Tap an option to check your answer0 / 4
Q1.A Venn diagram uses ___ to show sets.
Explanation: Overlapping circles.
Q2.The intersection of two sets contains elements that are:
Explanation: Common to both.
Q3.The union of two sets contains:
Explanation: All elements.
Q4.If $A=\{1,2,3\}$ and $B=\{3,4\}$, then $A\cap B=$
Explanation: Common element $3$.