How do I write Oracle SQL code to perform a simple math calculation??

with criteria as (select to_date('03/14/08', 'mm/dd/rr')  as start_date,
                         4 as periods,
                         'Week' as period,
                         200 per_period from dual),
periods as (select 'Week' period, 7 days, 0 months from dual
  union all select 'BiWeek', 14, 0 from dual
  union all select 'Month', 0, 1 from dual
  union all select 'ByMonth', 0, 2 from dual
  union all select 'Quarter', 0, 3 from dual
  union all select 'Year', 0 , 12 from dual
),
t1 as (
select add_months(start_date,months*(level-1))+days*(level-1) start_date,
       per_period,
       c.period||' '||level||' of '||c.periods period,
       per_period * 0.2 Federal_Taxes,
       per_period * 0.0765 FICA_Taxes,
       per_period * 0.7235 Net_Amount
  from criteria c join periods p on c.period = p.period
 connect by level <= periods)
select case grouping(start_date)
            when 1 then 'Total'
            else to_char(start_date)
       end start_date, 
       sum(per_period) per_period, 
       period
from t1
group by rollup ((start_date, period))

with criteria as (select to_date('03/14/08', 'mm/dd/rr')  as start_date,
                         4 as periods,
                         'Week' as period,
                         200 per_period from dual),
periods as (select 'Week' period, 7 days, 0 months from dual
  union all select 'BiWeek', 14, 0 from dual
  union all select 'Month', 0, 1 from dual
  union all select 'ByMonth', 0, 2 from dual
  union all select 'Quarter', 0, 3 from dual
  union all select 'Year', 0 , 12 from dual
),
t1 as (
select per_period * 0.2 Federal_Taxes,
       add_months(start_date,months*(level-1))+days*(level-1) start_date,
       per_period,
       c.period||' '||level||' of '||c.periods period,
       per_period * 0.0765 FICA_Taxes,
       per_period * 0.7235 Net_Amount
  from criteria c join periods p on c.period = p.period
 connect by level <= periods)
select case grouping(start_date)
            when 1 then 'Total'
            else to_char(start_date)
       end start_date, 
       sum(per_period) per_period, 
       period
from t1
group by rollup ((start_date, period))

with criteria as (select to_date('03/14/08', 'mm/dd/rr')  as start_date,
                         4 as periods,
                         'Week' as period,
                         200 per_period from dual),
periods as (select 'Week' period, 7 days, 0 months from dual
  union all select 'BiWeek', 14, 0 from dual
  union all select 'Month', 0, 1 from dual
  union all select 'ByMonth', 0, 2 from dual
  union all select 'Quarter', 0, 3 from dual
  union all select 'Year', 0 , 12 from dual
),
t1 as (
select add_months(start_date,months*(level-1))+days*(level-1) start_date,
       per_period,
       c.period||' '||level||' of '||c.periods period,
  from criteria c join periods p on c.period = p.period
 connect by level <= periods)
select case grouping(start_date)
            when 1 then 'Total'
            else to_char(start_date)
       end start_date, 
       sum(per_period) per_period, 
       period,
       sum(per_period) * 0.2 Federal_Taxes,
       sum(per_period) * 0.0765 FICA_Taxes,
       sum(per_period) * 0.7235 Net_Amount
from t1
group by rollup ((start_date, period))