Average time to read a single sector (disk)

We need to know:

Seek time: 12 ms (given in text)

Average rotational delay: 3600 RPM = 60 revs per second, or 16.7 ms/rev. Average is half that, or 8.3 ms.

Transfer time: 512 / 3.5 MB/s = 146 µs -- however, it will take a little longer to actually read the sector, since there are 64 sectors on the disk, and the disk rotates once every 16.7 ms, each sector will take 16.7 ms / 64 = 260 µs to read. Since this is greater than the transfer time, it is the figure to use.

Overhead: 5.5ms (given in text)

Disk Access time:

Seek time (12 ms) + Average rotational delay (8.3 ms) + Sector read time (260 µs) + Overhead (5.5 ms) = 26.1 ms

It is possible, depending on how the disk controller works, the sector may be read into a hardware buffer (260 µs) and then transferred to memory (146 µs) as two separate operations, in which case the answer would be 26.2 ms instead.