I have a spreadsheet that has (among many other things) three columns of data
for one farm. (see abbreviated example below)  One is a field identifier, the
next the practice occuring on that field, and the next the size of the field.
At the end of the ACRES column, there is a total of acres for the whole
farm.  I want a function that will identify all of the same practice and give
me the total of acres for that practice so that I can find what percentage of
the practice is occuring over the whole farm.  For example, I want a function
that will identify all the 441 practices and tell me that there are 19.8
acres of that practice in total.  I have tried VLOOKUP to find 441 and return
values in the 3rd row, but I could not figure out how to find all the 441's
and return the total of acres for that practice.  Any ideas?

FIELD                 PRACTICE         ACRES            
a                        441                       2.1
b                        442                     13.2                
c                        443                     12.8
d                        441                       0.5
e                        441                     17.2
f                         443                     11.1
                                 TOTAL         56.9