Boil Times and LME addition

I am reading Palmer. I've got a couple questions please.

If I understand correctly:

1st
For a partial boil.
He talks about adding 1 can or half your LME to reduce the OG of the wort for hop utilization, giving you the ability to use less hops with the same result?


2nd
I am a little confused on the boil times and hop additions.
Is it a standard 60min boil where you add the hops at certain times during that boil?

Or is it:
60min for hop X
30min for hop Y
15min for hop Z

Giving you 1hour 45 min boil?

60 minutes to go in a 60m minute boil, 30 to go in the boil, 15 to go in the boil.