manipulating a column in excel based on another column

i have two columns

a' column is a new monthly read value compared to the previous read which is in column b'
example
                           a                                      b
100 (gallons feb Usage)                        120 (gallons march usage)

unfortunately the two reading systems mutlipliers by gallons were not identical   (one system read in feb, a different one ran in march)

so system a might read 100 gallons as '1'   where as system b might read 100 gallons as '100'

I need excel to compare the value in column a to column b and create and manipulate column a's value.

example

a                                    b                              c(newly created value by adding two zero's based on comparing a & b)
100                             120000                     100000                      (it took the extra two zero's it identified by comparing a to b and added them
                                                                                                         to column a's value and placed the result in c.


does this make any sense to anyone?