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Answers to brain stretchers

(If you haven't already figured out the answers, you're cheating!  I'll probably have to fix your amp one of these days!)

Five 8-ohm speakers would have a total impedance of 8/5 or 1.6 ohms.
Six 8-ohm speakers would have a total impedance of 8/6 or 1.333 ohms.
Seven 8-ohm speakers would have a total impedance of 8/7 or 1.14 ohms.
Eight 8-ohm speakers would have a total impedance of 8/8 or 1.0 ohms.
I'll leave it up to you to figure out what happens with 4 ohm speakers.

Equivalents for a combination of a 4, 8, and 16 ohm speaker should use the 16 ohm speaker as the reference.
One 8 ohm speaker would be the equivalent of two 16 ohm speakers.
Since one 4 ohm speaker is the equivalent of two 8 ohm speakers, it is also equivalent to FOUR 16 ohm speakers.
So, 4 ohm and 8 ohm and 16 ohm equivalents are:
4(16s) and 2(16s) and 1(16) = 7(16s) in parallel, or 16/7 ohms = 2.286 ohms (same as we got the first time!)

A "Minimum 2 ohms" load could be:
Two 4 ohm speakers (4/2)
Four 8 ohm speakers (8/4)
One 4 ohm and two 8 ohm speakers - 2(8s) and 2(8s) =4(8s) =8/4 =2 ohms
and many other combinations

The following chart may help to remember. 
(The symbol "//" means "in parallel with".)

                             2
 =            4            //           4
 =    8     //   8               8    //   8
= 16//16  16//16       16//16  16//16