This one needs three components.
1. Number of tracks
There are $2400\ \mathrm{cylinders}$ with $6\ \mathrm{tracks\ per\ cylinder}$
So, $2400\times 6=14,400\ \mathrm{tracks}$
2. Time to read all tracks
A complete track can be read in one full rotation $:30\ \mathrm{ms}$
$\therefore 14,400\times 30=432,000\ \mathrm{ms}=432\ \mathrm{s}$
3. Cylinder-to-cylinder seek time
After finishing the six tracks in one cylinder, the head moves to the adjacent cylinder.
With $2400$ cylinders, there are:
$2400-1=2399$ adjacent-cylinder movements.
Each takes $20\ \mathrm{ms}$
$\therefore 2399\times 20=47,980\ \mathrm{ms}=47.98\ \mathrm{s}$
The initial positioning can require the stated worst-case seek:
$80\ \mathrm{ms}=0.08\ \mathrm{s}$
Hence, total time:
$432+47.98+0.08= \boxed{480.06\ \mathrm{s}}$
Answer : B
