SCOTT@orcl> column summation format a15
SCOTT@orcl> with
  2    test_data as
  3  	 (select '15.10' var1, '15.20' var2 from dual
  4  	  union all
  5  	  select '15.10' var1, '15.40' var2 from dual)
  6  select var1, var2,
  7  	    int+trunc(dec/45)||'.'||mod(dec,45) summation
  8  from   (select var1, var2,
  9  		    trunc(var1)+trunc(var2) int,
 10  		    substr(var1,instr(var1,'.')+1)+substr(var2,instr(var2,'.')+1) dec
 11  	     from   test_data)
 12  /

VAR1  VAR2  SUMMATION
----- ----- ---------------
15.10 15.20 30.30
15.10 15.40 31.5

2 rows selected.

ALTER TABLE MY_TABLE ADD (
  CONSTRAINT BAD_NUMBER
  CHECK (regexp_like(my_qty,'^-?\d*\.?\d*$'))
  ENABLE VALIDATE);

select to_char(sum(to_number(my_qty)),'FM9999999999990.00') FROM MY_TABLE;

before you add the constraint, you can check the data by running the following query

select * from my_table
where not regexp_like(my_qty,'^-?\d*\.?\d*$');

The regular expression allows nulls in the column. If you don't want nulls, make the column not null;