Explanation : In 60 seconds, the disk rotates 2400 times. So it takes 25ms to rotate it one time.
There are 200 tracks. On average, 100 tracks need to be traveresed to reach a specific track.
So 25ms*100 tracks= 2.5s time is taken on average to reach a particular track
Explanation : In 25 ms, one rotation is completed and all the 62500 bits are transferred. So in 1ms, 2500 bits are transferred. Accordingly, in 1s, 2.5Mb are transferred. Thats why the speed is 2.5Mb/s
MB - Mega Bytes,