As the hard disk makes 360 revolutions per second, so one full rotation requires 1/360 sec = 0.002778 sec = 2.778 ms.
On average, however, we expect to require half a rotation. Therefore, we need 1.389 ms. The second part of the movement depends on the seek time which is 10 ms. Therefore, the total time is 1.389+10=11.389 ms on average.
