In the first pass we can create [105/5]=21 sorted sub-files,each 5 pages long.
In the second pass by 4-way merging (4 because one of the 5 available buffers has to be reserved for holding the output),
We can create [21/4]=6 sorted sub-files,each 20 pages long (except the last).
Again applying 4-way merge sort,we get create [6/4]=2 sorted sub-files. These two can be merged to get the final sorted file. We need a total of 4 passes. Total cost will be 2 x 105 x 4=840 units.
Hence obtion C.
