Which number breaks the pattern? 3, 6, 11, 20, 33, 70

Correct — A.
Rule: each term = previous term × 2, minus 0, 1, 2, 3, 4 in turn.

3 × 2 − 0 = 6 ✓
6 × 2 − 1 = 11 ✓
11 × 2 − 2 = 20 ✓
20 × 2 − 3 = 37, but 33 is printed ✗

37 × 2 − 4 = 70 ✓, so 70 fits once 37 is restored.
33 breaks the pattern → option (a).

Why the others are wrong

• (b)
  70 — 70 fits the rule: 37 × 2 − 4 = 70. It looks wrong only beside 33, because the error sits in the term before it.
• (c)
  11 — 11 = 6 × 2 − 1, the rule's third term exactly, and 11 × 2 − 2 gives the printed 20 next.
• (d)
  20 — 20 = 11 × 2 − 2, exactly as the rule predicts, and 20 × 2 − 3 = 37 leads on correctly to 70.

In a wrong-term series one printed number has been altered. Find a rule that fits the terms around it, then test every term; the one that fails is the answer.

The differences expose it here. With 37 restored they run 3, 5, 9, 17, 33, each one double the last minus 1. With 33 printed they run 3, 5, 9, 13, 37 and break at the fourth.

Key facts

• The corrected series is 3, 6, 11, 20, 37, 70.
• Its differences 3, 5, 9, 17, 33 each equal double the previous difference minus 1.
• A wrong term inside a series spoils two differences, the one before it and the one after.

Common traps

• Blaming 70 because it makes the biggest jump, when 37 × 2 − 4 = 70 fits
• Reading the differences 3, 5, 9, 13 as a settled pattern and blaming the next term

No directly related past PYQ was found.