How to find percentage of CPU time consumed during DMA operation?
1 word=64 bit=8 B

64 B=8 word

machine cycle time=2 microseconds => 1 word is transferred from io buffer to memory by cpu in 2 microseconds =>  data preparation time by cpu 
64 B data and io bandwidth is 1MBps

so 64/10^6 seconds = 64 microseconds = transfer time by io device = time to dump data from io device to io buffer

now in cycle stealing mode generally data is transferred byte wise..but here word size is given so we assume data is transferred word wise..

64B data in 64 microseconds

=> 8 word data in 64 microseconds

=> 1 word data in 8 microseconds = this is word transfer time by io to buffer

so % cpu blocked= % cpu time consumed = % time CPU is actually transferring data from io buffer to main memory= (2/8)*100%=25%

in burst mode its ( 2 / (2+8) )*100%=20%>>>>>>>

in burst mode all data is transferred once as a block...so we wait for io to dump all of 64 B data in its buffer which is 64 microseconds

now cpu will fully transfer this data from io buffer to main memory..64 B data meaning 8 word data meaning 8*2 microseconds meaning 16 microseconds..

so  16*100/(16+64) % = 2*100/(2+8) = 20%