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ISRO CS 2025 Q.68: solution and answer

ISRO 2025 · CSQ.68 · 1 mark · MCQ

68. In the context of Operating system process scheduling, if W represents Wait Time, R represents Run Time, T represents Turnaround time, S represents Response time, then which of the following is correct option with respect to given diagram?

ISRO CS 2025 question 68: A horizontal time axis (arrow labelled 'time' at right) with four vertical tick marks labelled, left to right, Arrival…Tap to open full size
Figure from the ISRO CS 2025 paper. It is part of the question.
Figure description

A horizontal time axis (arrow labelled 'time' at right) with four vertical tick marks labelled, left to right, Arrival, Execution, First Output, Finish. A box labelled 'Job Execution' is drawn above the axis spanning from the Arrival tick to the Finish tick. Below the axis are four horizontal bars: Time1 spans Arrival to Execution; Time2 spans Execution to Finish; Time3 spans Arrival to First Output; Time4 spans Arrival to Finish.

  1. (A)

    Time1:W, Time2:R, Time3:T, Time4:S

  2. (B)

    Time1:S, Time2:W, Time3:R, Time4:T

  3. (C)

    Time1:W, Time2:R, Time3:S, Time4:T

  4. (D)

    Time1:R, Time2:S, Time3:W, Time4:T

Answer and solution

ANSWER

Option (c): Time1 is wait time (W), Time2 is run time (R), Time3 is response time (S) and Time4 is turnaround time (T).

Official answer: CSource: ISRO's final answer key of 15 May 2026

  1. Read each bar by the two marks on the time axis where it starts and ends. The marks are Arrival, Execution, First Output and Finish.

  2. Time1 runs from Arrival to Execution. This is the time the job waits before it first gets the CPU, so it is the wait time (W).

  3. Time2 runs from Execution to Finish. This is the time the job spends executing, so it is the run time (R).

  4. Time3 runs from Arrival to First Output. Response time is the gap between arrival and the first response, so it is the response time (S).

  5. Time4 runs from Arrival to Finish. Turnaround time is finish time minus arrival time, so it is the turnaround time (T).

  6. The mapping is Time1:W, Time2:R, Time3:S, Time4:T, which is option (c).

  7. Option (a) calls Time3 turnaround and Time4 response. This is wrong because Time4 covers the whole stay in the system, which is the turnaround time. Option (b) calls Time1 response. This is wrong because Time1 ends at the start of execution, and the first output comes later. Option (d) calls Time1 run time. This is wrong because during Time1 the job is only waiting.

KEY IDEA

Name each interval by its two end marks. Arrival to first CPU is wait. First CPU to finish is run. Arrival to first output is response. Arrival to finish is turnaround.

WATCH OUT FOR

Mixing up response time and turnaround time. Both start at arrival, but response time stops at the first output and turnaround time stops at the finish. Another slip is to read the wait time as the response time, although the first output comes after execution starts.

From the ISRO CS 2025 paper. Every question of this paper · The full paper (PDF, ISRO) · All 3 ISRO CPU scheduling questions, with notes