Letter, Number and Series Notes for SSC Exams

Letter, number and series questions are asked in every SSC Reasoning paper. You are given a few terms of a number series or an alphabet series, and you have to find the next term, the missing term or the wrong term. Marks in this chapter depend on how quickly you can spot the pattern. These number series notes for SSC explain every pattern that is repeated in CGL, CHSL, MTS and GD exams, with tables, solved examples and practice questions.

Types of Series Questions

• Next term: find the term that comes after the given terms.
• Missing term: one term in the middle is replaced by a question mark.
• Wrong term: one term does not follow the pattern and you have to find it.
• Alphabet or letter series: the terms are letters or letter groups.
• Alphanumeric series: letters and numbers are mixed, and each part has its own pattern.

Number Series: Common Patterns

Squares and Cubes to Remember

Prime numbers up to 40: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37.

How to Solve a Number Series

1. Find the difference between every two terms. If the differences are the same, it is an arithmetic series.
2. If the differences keep changing, find the difference of the differences.
3. If the terms grow very fast, try multiplication, for example ×2, ×3 or ×2 + 1.
4. If the terms look like squares or cubes, compare them with the tables above.
5. If nothing works, split the series into odd-place and even-place terms.

Example 1. Find the next term: 2, 6, 12, 20, 30, ?
Differences are 4, 6, 8, 10, so the next difference is 12. Answer: 42.

Example 2. Find the next term: 7, 10, 16, 28, ?
Differences are 3, 6, 12, so the next difference is 24. Answer: 52.

Example 3. Find the next term: 2, 3, 8, 27, 112, ?
The pattern is ×1 + 1, ×2 + 2, ×3 + 3, ×4 + 4. So the next is 112 × 5 + 5. Answer: 565.

Example 4. Find the next term: 2, 5, 4, 8, 6, 11, 8, ?
Odd-place terms are 2, 4, 6, 8 and even-place terms are 5, 8, 11, 14. Answer: 14.

Wrong Number Series

In these questions, one term breaks the pattern. First find the pattern from the terms that look correct, then check every term against it.

Example 5. Find the wrong term: 2, 5, 10, 17, 26, 37, 50, 64.
The pattern is n² + 1, so the terms should be 2, 5, 10, 17, 26, 37, 50, 65. Wrong term: 64.

Example 6. Find the wrong term: 4, 8, 16, 33, 64, 128.
Every term is double the previous one, so 33 should be 32. Wrong term: 33.

Alphabet (Letter) Series

Convert letters to their positions (A = 1 to Z = 26) and then find the pattern. Remember that the alphabet repeats after Z.

Example 7. Find the next group: ACE, FHJ, KMO, ?
Each group has letters with a gap of one letter: A, C, E; then F, H, J; then K, M, O. The next group starts with P. Answer: PRT.

Quick Tips to Solve Faster

• Check the difference first, then ratio, then squares and cubes.
• If the series has a very large jump, think of multiplication or cubes.
• If two terms in a row are the same, think of Fibonacci or alternate series.
• In letter series, write the position numbers below the letters to see the pattern clearly.
• In wrong-term questions, do not trust the first and last term blindly. Test the pattern on at least three terms.

Quick Revision Points

• Arithmetic: same difference. Geometric: same ratio.
• Learn squares up to 20 and cubes up to 10.
• Prime numbers up to 40 are asked often.
• Alternate series means two patterns mixed together.
• In letter series, the alphabet is a circle: after Z comes A.

Practice Questions With Answers

1. 5, 9, 13, 17, ?
   Answer: 21 (+4 each time).
2. 2, 6, 12, 20, 30, ?
   Answer: 42 (differences 4, 6, 8, 10, 12).
3. 3, 6, 12, 24, ?
   Answer: 48 (×2 each time).
4. 1, 4, 9, 16, ?
   Answer: 25 (squares).
5. 2, 3, 5, 7, 11, ?
   Answer: 13 (prime numbers).
6. 7, 10, 16, 28, ?
   Answer: 52 (differences 3, 6, 12, 24).
7. J, L, N, P, ?
   Answer: R (+2 each time).
8. Find the wrong term: 3, 9, 27, 82, 243.
   Answer: 82 (it should be 81).
9. AB, DE, GH, JK, ?
   Answer: MN (each pair moves three places).
10. 1, 2, 6, 24, 120, ?
    Answer: 720 (multipliers 2, 3, 4, 5, 6).

How to Practise This Chapter

Solve ten series questions daily and write the pattern next to each answer. Then test your speed with timed sets in our practice papers, and see how series questions appear in actual exams on the previous year papers page. Revise Coding-Decoding before this chapter, and continue with Analogy after it.

Explore More SSC Subjects

SSC exams test many subjects, so connect this topic with the related subjects below:

• Quantitative Aptitude Notes
• English Notes
• General Awareness Notes
• Computer Knowledge Notes
• Stenography Notes
• All SSC subjects and notes

Frequently Asked Questions

What is a series in reasoning?

A series is a list of numbers, letters or a mix of both arranged according to a hidden rule. You have to find that rule and use it to find the next, missing or wrong term.

How many questions come from series in SSC exams?

Usually two to four questions come from number, letter or alphanumeric series in the Reasoning section of SSC Tier 1 papers, although the exact number changes every year.

What is the first thing to check in a number series?

Check the difference between consecutive terms. If the difference is constant, it is an arithmetic series. If not, check the ratio and then the difference of differences.

How do I identify a series based on squares and cubes?

Compare the terms with the squares and cubes you have memorised. If a term is close to a perfect square or cube, it is likely a square or cube plus or minus a small number.

What is an alternate series?

An alternate series has two separate patterns mixed together. Odd-place terms follow one rule and even-place terms follow another, so you should write them separately.

How do I find the wrong term in a series?

Find the pattern from the terms that seem correct, apply it to every term and mark the one that does not match. Test the pattern on at least three terms before deciding.

Which prime numbers should I remember?

Remember the primes up to 40: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31 and 37. These appear often in series questions.

How should I solve letter series quickly?

Convert each letter into its position number, find the pattern in the numbers and convert back to letters. Keep in mind that the alphabet starts again after Z.

What is an alphanumeric series?

It is a series with both letters and numbers, such as A1, C3, E5. Solve the letters and the numbers separately, because each part follows its own rule.

Can a series have more than one correct pattern?

In rare cases yes, but in exams only one pattern fits all the terms. Always verify your pattern on every term given.

How much time should I spend on one series question?

Try to solve it within 20 to 30 seconds. If you do not see the pattern in that time, mark the question and return to it after finishing the easier ones.

How can I improve my speed in series questions?

Memorise squares up to 20 and cubes up to 10, practise ten questions daily, and write the pattern after each solved question so that you start recognising it quickly.

Are series questions the same in Hindi medium papers?

Yes. The numbers and patterns are the same in Hindi and English papers. Letter series may use English letters in both versions.