Avg rotation latency :
$15,000\ rot\rightarrow60sec$
$1\ rot\rightarrow\ ?$
$1\ rot=4ms$
$\dfrac{1}{2}rot\rightarrow2ms$
$Avg\ Seek\ time=2\times Avg\ rotation\ latency$
$Avg\ Seek\ time=4ms$
$Data\ transfer\ time:$
$1\ sec\leftarrow 50\times 10^6B$
$?\leftarrow 512B$
$0.0102ms$
$Controller's\ transfer\ time=10\times data\ transfer\ time$
$Controller's\ transfer\ time=0.102ms$
Total$=6.1ms$