explain how did we get

b $=$ time to transfer one byte from the disk surface to the disk controller, in milliseconds

b $=($time to transfer $512$ bytes$)/(512$ bytes$)=(0\mathrm{.}01$ ms$)/(1$ byte$)=0\mathrm{.}00001$ ms$/$byte

how do we know that the time to transfer $512$ bytes is $0\mathrm{.}01$ ms$????$