Questions
2–4 questions per paper
Difficulty
Medium
Importance
High-yield baseline for SSC and banking quant sections
Overview
The Number System serves as the foundational pillar for all quantitative aptitude sections in SSC and banking examinations. Mastering these concepts allows for rapid mental calculations and provides the logic necessary to solve complex arithmetic and algebraic problems efficiently.
Divisibility Rules
Divisibility rules are essential speed-solving tools that allow you to determine if a number is divisible by a divisor without performing the actual long division. Memorizing these rules for 2 through 13 is critical for quickly identifying factors or checking options in multiple-choice questions.
- Divisibility by 3/9: Sum of digits must be divisible by 3/9.
- Divisibility by 4: Last two digits must be divisible by 4.
- Divisibility by 8: Last three digits must be divisible by 8.
- Divisibility by 11: Difference between the sum of alternate digits must be 0 or a multiple of 11.
- Divisibility by 7: Subtract double the last digit from the remaining number; result must be divisible by 7.
HCF & LCM
HCF (Highest Common Factor) and LCM (Least Common Multiple) problems frequently appear in the context of bells ringing, race tracks, or finding the greatest number that divides various sets. Understanding the relationship between two numbers and their HCF/LCM is a high-frequency exam concept.
- Product of two numbers = HCF × LCM.
- HCF of fractions = (HCF of numerators) / (LCM of denominators).
- LCM of fractions = (LCM of numerators) / (HCF of denominators).
- HCF is always a factor of the LCM.
- Co-prime numbers have an HCF of 1 and an LCM equal to their product.
Simplification & BODMAS
Simplification questions test your ability to follow the correct order of operations, preventing errors in multi-step arithmetic. Precision and speed are gained by recognizing common squares, cubes, and fraction-to-percentage conversions.
- Order: Brackets, Of, Division, Multiplication, Addition, Subtraction.
- Always solve inner-most brackets first.
- 'Of' takes precedence over multiplication.
- Use digital sum method to verify arithmetic results quickly.
- Memorize squares up to 30 and cubes up to 20 for rapid calculation.
Formula Sheet
Product of two numbers = HCF(a,b) * LCM(a,b)
HCF(a/b, c/d) = HCF(a,c) / LCM(b,d)
LCM(a/b, c/d) = LCM(a,c) / HCF(b,d)
Exam Tip
When faced with complex simplification, check the units digit or perform a digital sum analysis to eliminate options rather than solving the entire expression.
Common Mistakes
- Applying the BODMAS order incorrectly by performing multiplication before division or addition before multiplication.
- Confusing HCF and LCM formulas in word problems involving time intervals or grouping items.
- Failing to check the divisibility rule of 7 or 11 carefully, often leading to sign errors.
More Revision Notes
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