Series (Number & Letter)
🔒 Log in to trackSeries questions ask for the next or missing term of a number or letter sequence, or the term that spoils a given sequence. CGL asks 4-5 per Tier 1 shift. The universal tool is the difference ladder; the recurring families are AP, ×k±r, squares/cubes, interleaved pairs and stepped letter tracks.
One page per subtopic: detailed notes, every question type, formulas, tricks and practice sets.
Every formula on one printable page, grouped by subtopic.
4 exam-level questions worked step by step.
95 questions — untimed practice or a timed test with analysis.
Track record in the exam
Questions per shift in recent SSC CGL papers.
Test difficulty mix (95 questions)
Question patterns exams keep repeating
Taken from previous-year papers. If a pattern is marked "very common", expect to see it in your exam.
Gaps that keep growing
very commonNumbers grow slowly and the gaps between them also grow (or shrink) by a fixed step.
Write the gaps between the numbers.
Continue the gaps, then add the next gap to the last number.
11, 13, 17, 23, 31, ?
Gaps: 2, 4, 6, 8
The next gap is 10
31 + 10 = 41
Multiply then add or subtract
very commonNumbers roughly double or triple each time.
Divide each term by the one before to guess the multiplier.
Find the small number added or taken away.
4, 10, 28, 82, ?
Each term is about 3 times the last
4 × 3 − 2 = 10 and 10 × 3 − 2 = 28
82 × 3 − 2 = 244
Squares and cubes
commonNumbers sit just next to squares or cubes (like 3, 8, 15, 24 or 2, 9, 28, 65).
Compare each number with the nearest square or cube.
Find the fixed number added or taken away.
2, 9, 28, 65, ?
Cubes are 1, 8, 27, 64
Each term is a cube + 1
Next cube is 5³ = 125, so 125 + 1 = 126
Changing pattern
commonThe growth is uneven, like ×1+1, ×2+2, or the series goes up and down.
Try odd and even places as two separate series.
Otherwise write the step between each pair.
2, 3, 8, 27, 112, ?
2 × 1 + 1 = 3
3 × 2 + 2 = 8, then 8 × 3 + 3 = 27, then 27 × 4 + 4 = 112
Next: 112 × 5 + 5 = 565
Missing number in the middle
very commonThe ? is between other numbers, not at the end.
Find the rule from the numbers on both sides.
Check that the next number also fits.
5, 8, 14, ?, 50, 98
5 × 2 − 2 = 8 and 8 × 2 − 2 = 14, so the rule is × 2 − 2
Check the right side: 26 × 2 − 2 = 50 ✓
? = 14 × 2 − 2 = 26
Letter series
very commonLetters or groups of letters follow a step (like BDF, GIK).
Change letters to numbers (A = 1 ... Z = 26).
For groups, treat each place as its own series.
AZ, CX, EV, GT, ?
First letters: A, C, E, G go up by 2, next is I
Second letters: Z, X, V, T go down by 2, next is R
Answer: IR
Letters and numbers together
commonEach term mixes a letter and a number (like B2, D4, F8).
Split the terms into a letter series and a number series.
Solve each one and join the answers.
B2, D4, F8, H16, ?
Letters: B, D, F, H go up by 2, next is J
Numbers: 2, 4, 8, 16 double, next is 32
Answer: J32
Find the wrong number
commonNo ? is given. The question asks which number is wrong.
Find the rule from the first few numbers.
Rebuild the series and spot the one that does not fit.
2, 6, 18, 54, 164, 486. Which number is wrong?
The rule is × 3
54 × 3 = 162, not 164
162 × 3 = 486 fits, so the wrong number is 164
Fill blanks in a letter pattern
very commonA line of letters with blanks, like "a _ c a b _ a b c _ b c".
Guess the block that repeats.
Write the full pattern and fill each blank.
Fill the blanks: a _ c a b _ a b c _ b c
The block abc repeats
Full line: abc abc abc abc
Blanks in order: b, c, a, so bca