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[img=http://img177.imageshack.us/img177/4127/circuitcz2.th.gif]

The problem asks for the current in the interconnecting wire between the 2 wires with the 2 resistors.

I solved this using loop rules, but the book shows a shortcut by using equivalent resistance. The book states the top 10 ohm and 20 ohm resistors are parallel and the bottom two are in parallel as well. However, I am confused as how this is possible. There are other paths between the start and end node points of the 10 and 20 ohm resistors. How can they be in parallel?

I was thinking this was a special case maybe, because normally they shouldn't be parallel, but the symmetry of the arrangement means the voltage at end points of the resistor pairs are the same, hence making them essentially parallel.