Find the next number in the series: 885, 870, 855, 840, _____?

Appeared in: RRB Staff Nurse Delhi-2015

Explanation

The question asks to identify the next number in a given arithmetic sequence.
The series follows a simple subtraction pattern, where each term is 15 less than the previous term.
To find the next term, subtract 15 from the last known term: 840 - 15 = 825.
This type of series is known as an arithmetic progression with a common difference of -15.

Option A: This option results from incorrectly subtracting 5 from the last term (840 - 5 = 835), instead of the correct common difference of 15.
Option B: This is the third term in the series. A decreasing series cannot have a later term that is equal to an earlier term.
Option D: This option results from incorrectly subtracting 25 from the last term (840 - 25 = 815), which does not match the series' pattern.

Visual 1: Flowchart - A simple flowchart showing the sequence of numbers with arrows indicating '-15' between each step: 885 → (-15) → 870 → (-15) → 855 → (-15) → 840 → (-15) → 825. This visually reinforces the consistent subtraction pattern.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Number Series and Pattern Recognition as background academic context rather than a clinical decision trigger.
While not a clinical question, pattern recognition is a crucial nursing skill. Nurses must identify trends in vital signs, lab results, or patient symptoms to detect improvement or deterioration.
For example, recognizing a consistent drop in blood pressure over several readings (a pattern) allows for timely intervention.
What if the series was 885, 870, 860, 855? The pattern would no longer be a simple arithmetic progression, requiring a more complex analysis of alternating or changing differences. This highlights the need for careful observation rather than assumption.

How to Approach the Question

First, read the series carefully: 885, 870, 855, 840.
Calculate the difference between the first two numbers: 885 - 870 = 15.
Verify if this difference is consistent. Check the next pair: 870 - 855 = 15. And the next: 855 - 840 = 15.
Since the difference is a constant subtraction of 15, you have identified the rule of the series.
Apply this rule to the last term to find the missing number: 840 - 15.
Calculate the result (825) and select the matching option.

Concept Tested & Keywords

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Practise the full RRB Staff Nurse Delhi-2015

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