Find the missing number in the following number series.
-12, -6, __, 9, 18

Appeared in: DSSSB - 29 August 2019 (Shift-1)

Explanation

The core concept is identifying the pattern in a numerical sequence, which is a fundamental aspect of logical reasoning.
The pattern here is not a simple arithmetic or geometric progression but involves adding an incrementally increasing value at each step.
The first jump is by +6 (-12 to -6). The last jump is by +9 (9 to 18). This suggests the intermediate jumps are +7 and +8.
Applying this logic: -12 + 6 = -6; -6 + 7 = 1; 1 + 8 = 9; 9 + 9 = 18.
The sequence of additions (+6, +7, +8, +9) confirms that the missing term must be 1.

Option A: If the missing number were 0, the series of additions would be +6, +6, +9, +9. This pattern is not a simple arithmetic progression of differences.
Option C: Choosing 2 would create a difference pattern of +6, +8, +7, +9. The sequence of additions lacks a logical order, making this option incorrect.
Option D: If -1 were the missing term, the differences would be +6, +5, +10, +9. This sequence of additions is erratic and does not follow any discernible logical rule.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Number Series as background academic context rather than a clinical decision trigger.
This question tests logical reasoning and pattern recognition, which are essential cognitive skills for nurses.
Nurses use pattern recognition to identify changes in a patient's condition, interpret trends in vital signs, and recognize adverse drug reactions.
Strong analytical skills help in making sound clinical judgments and prioritizing patient care effectively.

How to Approach the Question

First, identify the type of question. This is a 'find the missing number' in a series problem.
Calculate the difference between the known consecutive terms to look for a primary pattern. (-6 - (-12) = +6) and (18 - 9 = +9).
Observe the differences you've calculated (+6 and +9). Notice that there are two missing steps between them.
Formulate a hypothesis. The simplest logical progression from a difference of +6 to +9 over two steps is an arithmetic sequence: +6, +7, +8, +9.
Test your hypothesis. Start with the first term and apply the sequence of additions: -12 + 6 = -6 (Correct). -6 + 7 = 1. 1 + 8 = 9 (Correct). 9 + 9 = 18 (Correct).
Since the hypothesis works for the entire series, the missing number is 1. Select the corresponding option.

Concept Tested & Keywords

QyTceSIHK38Ze-I0zrBFIY

Practise the full DSSSB - 29 August 2019 (Shift-1)

Attempt every question from this paper in a timed mock, then review the full solution for each one.