use SUM and date difference in the datagrids with a dataset

using windows application.

first form details:  two datagrids--first for main table, second for sub table (they are related in dataset)
how do I sum the column containing rows of numbers into a single row of total amount in the "sub table" datagrid ?

second form:  same as above.  
how do I set up the date difference--"sub table" datagrid with three columns (col 1 - previous date, col 2 - current date, col 3 - date difference by DAYS)

the data comes from a xml file and is stored in the dataset.  I don't know if i need to do something with xml schema...

hope these details help...thanks