Number Series and Alphabet Series for the Assam Police Constable (AB & UB) Exam
Number Series and Alphabet Series belong to the reasoning paper of the SLPRB Assam Police Constable (AB & UB) examination and are asked to test how quickly a candidate can find the rule hidden inside a list. In a number series a set of numbers is given and the next number or the missing number must be found. In an alphabet series the same is done with letters. The questions carry one mark each and need no general knowledge and no long calculation, only the ability to spot the pattern. This page explains the common patterns, shows worked examples, gives shortcuts for speed, and lists the traps that quietly take away marks. The material follows the standard reasoning reference books used for Assam recruitment examinations and is reviewed before it is published.
What the Topic Means
A series is a list arranged according to a fixed rule. The candidate must discover that rule and then use it to supply the term that is asked. The rule may be a repeated addition, a repeated multiplication, a square, a cube, a mixed operation or an arrangement of the letters of the alphabet. Once the rule is found the answer is almost automatic, so the real skill is in looking at the differences between the terms, because the differences usually reveal the rule at once.
Common Number Series Patterns
- Difference series: each term is obtained by adding or subtracting a fixed number, as in 3, 7, 11, 15, where 4 is added each time.
- Increasing difference series: the difference itself grows, as in 2, 4, 8, 14, 22, where the differences 2, 4, 6, 8 increase by 2.
- Multiplication series: each term is multiplied by a fixed number, as in 2, 6, 18, 54.
- Square and cube series: the terms are squares or cubes of the natural numbers, as in 1, 4, 9, 16 or 1, 8, 27, 64.
- Mixed series: two operations alternate, as in 5, 10, 8, 16, 14, where multiplication by 2 and subtraction of 2 take turns.
- Two-tier series: two separate series are interleaved, as in 2, 3, 6, 5, 12, 7, where the odd places and the even places follow different rules.
- Prime number series: the terms are prime numbers, as in 2, 3, 5, 7, 11.
Common Alphabet Series Patterns
- Fixed skip: each letter is moved forward by a fixed number of places, as in A, D, G, J, where three steps are taken each time.
- Growing skip: the number of skipped places increases, as in A, C, F, J, where 1, 2, 3 places are skipped.
- Reverse skip: the letters move backward by a fixed number of places.
- Two interleaved series: the odd-position and even-position letters follow separate rules.
- Position-based series: the numerical position of the letter decides the next letter.
The Alphabet Positions
The alphabet has twenty-six letters and it helps to remember that A is 1, E is 5, J is 10, O is 15, T is 20 and Z is 26. A useful shortcut is the pair rule: A and Z add up to 27, B and Y add up to 27, and so on, so the position of a letter from the end is 27 minus its position from the beginning. The letters with the same sound, called the EJOTY group and the CFILORUX group, also help in writing positions quickly.
Worked Examples
First series: 4, 9, 16, 25. The terms are 2 squared, 3 squared, 4 squared and 5 squared, so the next term is 6 squared, that is 36. Second series: 3, 6, 12, 24. Each term is double the previous one, so the next term is 48. Third series: 20, 17, 14, 11. The terms fall by 3 each time, so the next term is 8. Fourth series: A, C, F, J. The skips are 1, 2 and 3 places, so the next skip is 4 places and the next letter is O.
How This Topic Appears in the Examination
The topic usually appears as two to four objective questions of one mark each. The common forms are to find the next term, to find the missing term in the middle, and to pick the wrong term from a series. Because the answer depends only on the rule, the questions are quick and are a good place to save time for the reasoning section.
Shortcuts and Speed Methods
- Write the differences between the terms in a row below the series; the rule often appears at once.
- If the differences do not help, write the differences of the differences.
- Check for squares and cubes when the numbers grow quickly.
- Look for a repeated multiplier when the terms grow in a fixed ratio.
- In a two-tier series, mark the odd places and the even places separately.
- For the alphabet, write the numerical positions below the letters.
Common Mistakes to Avoid
- Assuming an addition series when the numbers actually multiply.
- Ignoring the possibility of two interleaved series.
- Forgetting the reverse alphabet position when a question gives the position from the end.
- Mixing the English alphabet order by mistake.
- Rushing and missing that the difference itself is changing.
Preparation Plan
- Learn the common patterns listed on this page.
- Practise ten series a day and write the difference row for each.
- Memorise the alphabet positions and the pair rule.
- Solve the previous papers of Assam Police and related examinations.
- Revise the patterns once a week until they are recognised on sight.
More Practice Series
- 2, 6, 12, 20, 30, the next term is 42, since the terms are one more than successive squares.
- 1, 4, 27, 256, the terms are numbers raised to their own value, so the next is 3125.
- 5, 7, 10, 14, 19, the differences are 2, 3, 4, 5, so the next is 25.
- Z, X, V, T, the letters move back by two places each time, so the next is R.
- AZ, BY, CX, the first letter moves forward and the second moves backward, so the next is DW.
Quick Revision Checklist
- Difference, growing difference, product, square and cube series.
- Mixed and two-tier series.
- Alphabet skips that are fixed and that grow.
- The alphabet positions and the 27 rule for reverse positions.
- Always write the difference row first.
Frequently Asked Questions
What is a number series?
A number series is a list of numbers arranged by a fixed rule, in which the next term or a missing term has to be found.
How do I find the rule quickly?
Write the differences between the consecutive terms in a row. If the differences are constant the rule is addition; if they change steadily the difference is growing; if they do not help, write the differences of the differences.
What is a two-tier series?
A two-tier series is made of two separate series placed one after the other, so the odd places follow one rule and the even places follow another. They must be read separately.
How do I remember alphabet positions?
Remember that A is 1, E is 5, J is 10, O is 15, T is 20 and Z is 26. Also remember that the positions of opposite letters add up to 27, so the position from the end is 27 minus the position from the beginning.
Are number series and alphabet series important for Assam Police?
Yes. They appear in the reasoning paper with a few questions and are easy to score once the common patterns are learnt.