type conversion? - Mailing list pgsql-sql

All,

I have a column "total_hours  |  character varying(5) " and I am unable to
perform the aggrigate function SUM() because of the column type.
Without changing the type is there a way to convert the datatype by using
CAST(), to_char() or something simular? So that I may calculate the SUM().

SELECT SUM(total_hours) AS hours FROM my_table WHERE flag='$flag';

Thanks!
-p