Math and mySQL

Is there any way of retreiving the last record of a mySQL table where I may
specify one of the values in it?  I know the SELECT command and just for
example the field I know could be serial_number = 30612312-ABDEFG --- I have
the serial_number field set as a varchar.

Part two of my question is how do you do math in PHP?  I want to take the
running_total (another field in the table) and add or subtract what the user
has typed in.

Ron