merge sort

Question

Assume 5 buffer pages are available to sort a file of 105 pages. The cost of sorting using m-way merge sort is

Answer 1

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),one 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.