In missing-number questions, numbers are arranged in rows, columns or groups, and every group follows the same hidden rule. OPSC asks the group form, such as (24, 10, 392) and (18, 6, 216), in analogy questions too, so the method here helps in both topics.
Method
- Find the rule from the first complete row or group.
- Check it on the second row. A rule that fits only one row is a guess.
- Apply it to the row with the missing number.
Rules to test, in this order
- Sum: A + B + C.
- Product with a twist: A × B − C, A × B + C, A × C + B, (A × B) ÷ C.
- Bracket first: (A + B) × C, (A − B) × C, (A + C) × B.
- Squares: A² + B², A² − B², (A + B)², A² − B.
- Three numbers multiplied: A × B × C, or a × b × 2.
Example
(13, 3, 78), (14, 4, 112), (15, 6, ?). Try 13 × 3 = 39, and 78 = 2 × 39. Check row 2: 2 × 14 × 4 = 112 ✓. So the answer is 2 × 15 × 6 = 180.
Tips
- If the answer is much larger than the numbers given, think of multiplication or squares. If it is close to them, think of addition or subtraction.
- Use the options. Usually only one of them comes out of a simple rule.