If Elseif elseif problem

Hi

Im new to this and hope some of u guys/girls can help me.
i have some fields
D7=Starttid (format time)
D8=Sluttid (format time)
D9=Ferie (format decimal)
D10=Hour

What i have now is that D10=:
=IF([@Starttid]="";[@[Ferie]])
It say that if D7 is blank then D10=D9, that work, but what i want to do is something like this.
=IF((([@Sluttid]-[@Starttid])*24);IF([@Starttid]="";[@[Ferie]]))
but i cant get it to work.

What i need it to is:
If D7=blank then D10=D9 else D10= (D8 - D7)*24

can someone help me with a formel that can fix my problem !?

Maybe (in D10):

=HVIS(D7="";D9;(D8-D7)*24)

so easy....thx.

And in English Excel - in D10:

=IF(D7="",D9,(D8-D7)*24)