10 - compiles
5 - two test files (good and bad)
5 - good input file is a good set of test cases that exercise a good portion of the code (all?)
20 - struct or class for BigIntegers has good data members
20 - add BigIntegers function correct
carry the one
different lengths
20 - subtract BigIntegers function correct
borrowing
different lengths

10 - file I/O correct
5 - errors for invalid input 
5 - STYLE - other comments where appropriate (e.g. head of file, to clarify any tricky code), clear, meaningful variable names (I've been recommending camel case
such as numCorrect, etc), indentation/layout of code readable and regular

Extra credit possibilities (please record extra credit in a separate
column)
10-20 more complex than just struct or class with public data members
? - other things that really stand out in what they did ( try to give positive feedback too :-))