The circled part should be 50, as while the disk is preparing the 16B of data CPU is not blocked right? Now once the Preparation(storing the data in a data register is done) of 16B of data is done in 240nsec the next 50 nsec is used to transfer the data to the Memory by the DMA. So this 50nsec is when CPU is blocked.
So percentage of time CPU is blocked while DMA transfers 16B of data
= actual time / total time = (50/290)*100 = 17.24%