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Number System – Foundation of Quantitative Aptitude

📊 Quantitative Aptitude – Number System

 Types of Numbers

• Natural Numbers: 1, 2, 3…
• Whole Numbers: 0, 1, 2…
• Integers: -2, -1, 0, 1…
• Prime Numbers: Only 2 factors (2, 3, 5, 7)
• Composite Numbers: More than 2 factors

 Important Properties

• Even + Even = Even
• Odd + Odd = Even
• Even × Odd = Even
• Odd × Odd = Odd

 Divisibility Rules

• 2: Last digit even
• 3: Sum of digits divisible by 3
• 5: Ends in 0 or 5
• 9: Sum of digits divisible by 9
• 11: Alternate digit difference

 Square Tricks

Numbers ending with 5:

Formula: n × (n+1) + 25

• 25² = 625
• 45² = 2025
• 65² = 4225

 Practice Questions

Q1. Which number is divisible by 9?
A) 729   B) 734   C) 745   D) 752

Answer: 729

Q2. Find square of 85

Answer: 7225

Q3. Which is prime?
A) 21   B) 29   C) 35   D) 39

Answer: 29

 Speed Strategy

• Check options first
• Use divisibility rules
• Avoid full calculation
• Apply shortcut tricks

🎯 Practice Task

1. Find square of 75
2. Is 693 divisible by 9?
3. Which is prime: 31 or 33?

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