@ Puja Mishra first of all u have to go through the explanation given by arjun sir...he explained very nicely...And now i tried to make it more clear....
First Fit:
300--------> Chunk_350 bcz all the chunk before it are less than 300
now memory size becomes as
O_50 : F_150 : O_600 : F_50 : O_600, where O denotes occupied and F denotes free.
similarly,
25---------> F_150
memory size becomes as
O_50 : O_25 : F_125 : O_600 : F_50 : O_600
125-------> F_125
memory size becomes as
O_50 : O_25 : O_125 : O_600 : F_50 : O_600
50------> F_50
Best Fit:
300--------> Chunk_350 ( acc. to best fit, a block request is satisfied by the smallest block in that can fit it.)
now memory size becomes as
O_50 : F_150 : O_600 : F_50 : O_600,
similarly,
25---------> F_50
memory size becomes as
O_50 : F_150 : O_600 : O_25 : F_25 : O_600
125-------> F_150
memory size becomes as
O_50 : O_125 : F_25 : O_600 : O_25 : F_25 : O_600
50------> no space is available....so can't use it...