A man rows downstream at 15 km/hr and against the stream at 9 km/hr. His speed in still water is:

Appeared in: AIIMS Bhopal NO - 2018 (Shift-1st)

Explanation

The speed of the man in still water is the average of his downstream and upstream speeds.
The formula is: Speed in still water = (Downstream Speed + Upstream Speed) / 2.
Plugging in the given values: (15 km/hr + 9 km/hr) / 2.
Calculation: 24 / 2 = 12 km/hr.

Option A: This is an incorrect calculation. It does not correspond to any logical operation on the given speeds.
Option B: This is an incorrect calculation. It does not represent the speed in still water based on the provided data.
Option C: This is the downstream speed, which is the combined speed of the man and the stream. It does not represent the man's speed alone in still water.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain This question tests the application of relative speed concepts, specifically in the context of boats and streams, which is a common topic in general aptitude and mathematics sections of competitive exams as background academic context rather than a clinical decision trigger.
This question assesses general mathematical aptitude and problem-solving skills, which are essential for nurses.
While not a direct clinical calculation, the logical reasoning required is similar to that used in tasks like drug dosage calculations and interpreting patient data trends.
What if? If the speed in still water was 15 km/hr and the upstream speed was 9 km/hr, the stream's speed would be 15 - 9 = 6 km/hr, and the downstream speed would be 15 + 6 = 21 km/hr.

How to Approach the Question

First, identify the given information: Downstream speed (15 km/hr) and Upstream speed (9 km/hr).
Understand the goal: To find the speed of the man in still water.
Recall the basic formulas for boat and stream problems: Downstream speed = Boat's speed + Stream's speed; Upstream speed = Boat's speed - Stream's speed.
Set up two linear equations with two variables. Let 'x' be the boat's speed and 'y' be the stream's speed. So, x + y = 15 and x - y = 9.
Solve the system of equations. The simplest method here is to add the two equations to eliminate 'y', which gives 2x = 24.
Calculate the value of x: x = 12. This is the speed in still water.

Concept Tested & Keywords

Concept Tested: This question tests the application of relative speed concepts, specifically in the context of boats and streams, which is a common topic in general aptitude and mathematics sections of competitive exams.
Stem keywords: downstream, against the stream, speed in still water
Lead-in keywords: is

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