Correct : 7

To find the average time to read a random sector, we must calculate the total disk access time. This consists of three main components: Average Seek Time, Average Rotational Latency, and Transfer Time.

1. Average Seek Time
This is the time it takes for the read/write head to move to the correct track. The problem directly provides the average seek time. (Note: The adjacent track time of 2 ms is a distractor for a random sector calculation).
Average Seek Time = 5 ms.

2. Average Rotational Latency
This is the time it takes for the target sector to rotate under the read/write head. On average, this takes half of a full rotation.
First, find the time for one full rotation:
Rotational speed = 15000 revolutions per minute (rpm).
Revolutions per second = 15000 / 60 = 250 rps.
Time for 1 full rotation = 1 / 250 seconds = 0.004 seconds = 4 ms.
Average Rotational Latency = Half of a full rotation = 4 ms / 2 = 2 ms.

3. Transfer Time
This is the time it takes to actually read the data from the sector as it passes under the head.
Since one full rotation (4 ms) reads an entire track consisting of 400 sectors:
Transfer Time for 1 sector = 4 ms / 400 = 0.01 ms.

4. Total Average Access Time
Total Time = Average Seek Time + Average Rotational Latency + Transfer Time
Total Time = 5 ms + 2 ms + 0.01 ms = 7.01 ms.

The question asks for the answer as an integer in milliseconds. Rounding 7.01 gives us exactly 7.

Correct Answer: 7