SQL> with 
  2    data as (
  3      select 12.20 val from dual
  4      union all
  5      select 6.15 from dual
  6    ),
  7    total as (
  8      select sum(60*trunc(val)+100*mod(val,1)) total
  9      from data
 10    )
 11  select trunc(total/60)+mod(total,60)/100 result
 12  from total
 13  /
    RESULT
----------
     18.35

SQL> -- <= 24hrs
SQL> with
  2    data as (
  3      select 12.20 val from dual
  4      union all
  5      select 6.15 from dual
  6    ),
  7    total as (
  8      select sum(60*trunc(val)+100*mod(val,1)) total
  9      from data
 10    )
 11  select trunc(total/60)+mod(total,60)/100 result
 12  from total;

    RESULT
----------
     18.35

SQL> -- more than 24hrs OK?
SQL> with
  2    data as (
  3      select 22.20 val from dual
  4      union all
  5      select 6.15 from dual
  6    ),
  7    total as (
  8      select sum(60*trunc(val)+100*mod(val,1)) total
  9      from data
 10    )
 11  select trunc(total/60)+mod(total,60)/100 result
 12  from total;

    RESULT
----------
     28.35

SQL> -- using day fractions and dates handles >= 24hrs
SQL> with X as (select ((to_date('22.20', 'hh24.mi')-to_date('00:00','hh24.mi'))+
  2                     (to_date( '6.15', 'hh24.mi')-to_date('00:00','hh24.mi'))
  3                    ) dy_fract from dual)
  4  select TO_CHAR (TRUNC (dy_fract)) || ' day ' ||
  5           TO_CHAR (TRUNC (SYSDATE) + MOD (dy_fract, 1), 'HH24:MI') from X;

TO_CHAR(TRUNC(DY_FRACT))||'DAY'||TO_CHAR(TRUNC(SYS
--------------------------------------------------
1 day 04:35

SQL> -- or using Michel's more fancy format...
SQL> with X as (
  2      select '22.20' val from dual
  3      union all
  4      select '6.15' from dual
  5            ),
  6       Y as (select sum((to_date(val, 'hh24.mi')-to_date('00:00','hh24.mi'))) tot
  7             from X
  8            )
  9   select TO_CHAR (TRUNC (tot)) || ' day ' ||
 10          TO_CHAR (TRUNC (SYSDATE) + MOD (tot, 1), 'HH24:MI') from Y;

TO_CHAR(TRUNC(TOT))||'DAY'||TO_CHAR(TRUNC(SYSDATE)
--------------------------------------------------
1 day 04:35

SQL>

  1  with
  2	   data as (
  3	     select 36.75 val from dual
  4	     union all
  5	     select 6.35 from dual
  6	   ),
  7	   total as (
  8	     select sum(60*trunc(val)+100*mod(val,1)) total
  9	     from data
 10   )
 11   select trunc(total/60)+mod(total,60)/100 result
 12*  from total
SQL> /

    RESULT
----------
      43.5