Four wire ab, bc, cd and da each resistance 4 ohm and a fifth wire bd of resistance 8 ohm are joined to form a rectangle to which bd is a diagonal. the effective resistance between the points a and b is?

Since it's so late, and it's way past my bedtime, and I'm tired and hungry,
and I still have real work to do, and I don't think you're really a legitimate
asker of questions anyway, I'm going to do something that's against
my principles and I almost never do.  I'm going to violate my own personal
Prime Directive, and give you the answer without any explanation of where
it comes from.

Ready ?

Here goes:

The effective resistance between points 'A' and 'B' is (2 and 2/3) Ω .