Complete the series: 28, 42,59,79,102, ?

Correct — A.
Rule: the differences grow by 3.

Differences: 42 − 28 = 14, 59 − 42 = 17, 79 − 59 = 20, 102 − 79 = 23.
So 14, 17, 20, 23 → next difference 23 + 3 = 26.

Next term: 102 + 26 = 128 → option (a).

Why the others are wrong

• (b)
  157 — 157 would need a jump of 55 from 102. The differences climb by 3 (20, then 23), so the next is 26.
• (c)
  167 — 167 is 65 more than 102. The next difference is only 26, since 14, 17, 20, 23 grow by 3 at a time.
• (d)
  148 — 148 adds 46 to 102, double the last difference of 23. The differences grow by adding 3, not by doubling.

When a number series grows without an obvious multiplier, write the differences between neighbours under it: here 14, 17, 20, 23.

If those differences themselves move by a fixed amount (+3 here), extend the difference row first and then add. The next difference is 26, so the next term is 102 + 26 = 128.

Key facts

• 14, 17, 20, 23, 26 is an arithmetic progression with common difference 3.
• When first differences form an arithmetic progression, the second differences are constant: here every one is 3.

Common traps

• Adding the last difference 23 again instead of the next one, 26
• Stopping at the first differences without checking how they change

No directly related past PYQ was found.