If the data is equally distributed over 3 tracks, then you need 3 seeks. That will take 15ms. We want to read in 20ms, so we have 5ms left. We need the disk to make three complete rotations in those 5 milliseconds, one for each track. So the disk must do 3 rotations in 5 milliseconds. A minute is 60,000 milliseconds, so a disk that can do 3 rotations in 5 milliseconds can do 36,000 rotations in 60,000 milliseconds.
I don't know how 20,400 was calculated. I get 36,000 as shown above.