Series (Number & Letter)

What this question looks like

You see a line of small letters with some blanks, for example: a _ c a b _ a b c _ b c. You must choose the group of letters that fills the blanks, in order from left to right, so that the whole line follows a repeating pattern. The options look like 'bca', 'bac', 'cab'.

Think of a rangoli border: the same small design repeats again and again. Once you know one design unit, you can fill any gap in the border. SSC and Railway papers ask this type almost every shift.

Step 1: Find the block

Most such series repeat a small block of 3, 4, 5 or 6 letters. Count the total number of places (letters + blanks). If the total is 12, the block is often 3, 4 or 6 letters long; if it is 15, often 3 or 5. Try splitting the line into equal parts and see whether the visible letters agree.

Step 2: Fill using the same place in other blocks

Write the line in blocks, one below another. Each column must have the same letter. A blank takes the letter shown elsewhere in its column.

Example: a _ c a b _ a b c _ b c (12 places, block of 3)

• a _ c
• a b _
• a b c
• _ b c

Columns read a, b, c. The blanks are b, c, a → answer bca.

Step 3: Try the options when stuck

Put each option into the blanks and read the full line. The right option makes a clean repeat. This takes 10-15 seconds per option, so first cut options whose first letter clearly fails.

Pattern 1: fixed repeating block

The same block repeats: abc abc abc, pqrs pqrs pqrs, baab baab baab.

Pattern 2: growing block

Each block adds a letter: a, ab, abc, abcd (line: aababcabcd). Or letter counts grow: ab, aabb, aaabbb. Here the block length changes, so read the line as 1 + 2 + 3 + 4 places, or 2 + 4 + 6 places.

Pattern 3: mirror block

The block reads forwards and then backwards: pqr rqp pqr rqp. A doubled letter in the middle (rr, cc) marks the turning point. Another form walks up and down: a b c d c b a b c d c b a.

Pattern 4: rotating block

Each new block is the previous block with its first letter moved to the end: abcd, bcda, cdab, dabc. Or the last letter moves to the front: pqr, rpq, qrp. Split the line into equal blocks and check how each block is built from the one before.

Checks before you mark

• Count the blanks: the answer must have exactly that many letters.
• Fill in and read the whole line once. One wrong letter breaks the repeat.
• If two options both seem to fit, you have picked the wrong block size — try the next size.
• Blanks inside a word-like block (like 'baab') are easy to swap by mistake; fill column by column.

Quick revision

• Count total places → guess block size (3, 4, 5, 6).
• Write blocks one below another; columns must match.
• Four shapes: fixed block, growing block, mirror block, rotating block.
• A double letter (rr, cc) often marks a mirror turn.
• Always fill and re-read the full line.