A sailor goes 12km downstream in 48 minutes and returns in 1 hr and 20 minutes. The speed of the sailor in still water is:

Appeared in: RRB Staff Nurse Delhi-2015

Explanation

The first step is to convert the given times into a single unit, hours. Downstream time is 48/60 = 0.8 hours, and upstream time is 1 + 20/60 = 4/3 hours.
Next, calculate the speed for each part of the journey using the formula Speed = Distance / Time.
Downstream speed is 12 km / 0.8 hr = 15 km/hr.
Upstream speed is 12 km / (4/3) hr = 9 km/hr.
The speed of the sailor in still water is the average of the downstream and upstream speeds.
Calculation: (15 km/hr + 9 km/hr) / 2 = 12 km/hr.

Option B: This value is incorrect and likely results from a calculation error when averaging the upstream and downstream speeds.
Option C: This value is incorrect and likely results from a calculation error when determining the speeds or averaging them.
Option D: This is the calculated downstream speed (12 km / 0.8 hr = 15 km/hr). It represents the speed of the sailor with the help of the current, not the speed in still water.

Visual 1: Diagram - A simple diagram showing a boat on a river. One arrow indicates the direction of the boat and the stream (downstream), and another shows the boat moving against the stream (upstream). This can help visualize the concepts of 'u + v' and 'u - v'.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Calculating speed in still water using downstream and upstream speeds as background academic context rather than a clinical decision trigger.
This question does not test a clinical nursing concept but rather assesses general aptitude and mathematical skills.
Quantitative skills are important for nurses for tasks such as medication dosage calculation, interpreting lab results, and managing fluid balances.
What if the question asked for the speed of the stream? The speed of the stream would be (Downstream Speed - Upstream Speed) / 2, which is (15 - 9) / 2 = 3 km/hr.

How to Approach the Question

First, carefully read the problem and identify all the given information: distance, downstream time, and upstream time.
Notice that the time units are different (minutes and hours/minutes). Convert all time measurements into a single, consistent unit, such as hours.
Use the fundamental formula, Speed = Distance / Time, to calculate the speed for the downstream journey and the upstream journey separately.
Recall the relationship between speeds: Speed in still water is the average of the downstream and upstream speeds.
Substitute the calculated speeds into this formula to find the final answer.
Double-check your calculations and ensure the final answer matches one of the options.

Concept Tested & Keywords

Concept Tested: Calculating speed in still water using downstream and upstream speeds.
Stem keywords: downstream, upstream, speed in still water, 12km, 48 minutes, 1 hr 20 minutes
Lead-in keywords: speed of the sailor

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