Max density will be provided by the innermost track & the diameter of the innermost track is given as 10 cm

So let's calculate track capacity:

$\\ Track\ capacity=circumference\times density=\dfrac{22}{7}\times 10cm\times\dfrac{1400\ bits}{cm}=5500B$

$\\ \dfrac{1}{70}sec\rightarrow5500B\\ \\ 1sec\rightarrow 385000B\\ ?\leftarrow 64bit\\ ?=20.77\mu s$

$x$=$20.77\mu s$ is data transfer time

$y$=$1\mu s$ is transfer time

% of time CPU blocked=$\dfrac{y}{x}=\dfrac{1}{20.77}\times 100=4.81\%$ $\approx 5\%$