In a system containing cpu 1 and disk drive a, the system is instructed to access track 1, track 9, track 1, and then track 9 of drive a. the arm is initially located at track 1. drive a requires 35 ms for each move. without reordering, drive a requires to complete the access.
Drive A requires 105 miliseconds to complete the access.
Hard Drives actually have physical reading and writing arms inside them which are used for reading and writing data on the drives. To perform these actions they have to move from one location to another.
The example given in the question states that the arm is moving between track 1 and track 9. With it's initial position at track 1, it makes the same movement exactly 3 times from track 1 to track 9. If it requires 35 miliseconds for this movement, then drive A should require a total time of 105 miliseconds or 0.105 seconds to complete the given task.
I hope this answer helps.