SQL & PL/SQL

HI To all,

i'm trying to sum the total of the working hour for an entire week.
I've all (the interval monday / friday as well).

What i don't fully understand is how does extract works.
My prog stores the worked time in a col as interval day to second.

Provide me an example to do the sum in format hh:mm.
Let's only say that i've a table as follows:

COUNTER NOT NULL NUMBER
UTENTE VARCHAR2(30)
ORAINIZIOMATTINA TIMESTAMP(6)
ORAFINEMATTINA TIMESTAMP(6)
ORAFINEPOMERIGGIO TIMESTAMP(6)
TOTALE INTERVAL DAY(5) TO SECOND(6)
D DATE

I want only to output the sum the field TOTALE as hh:mm (togeter or separate as i can use 2 different field.

thanks a lot