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I'm trying to understand the second figure of the following webpage. The webpage simply solve it by 'symmetry', but I could not figure out its reasoning. Could someone explain how does it work? Thanks in advance.

http://www.schoolphysics.co.uk/age16-19/Electricity and magnetism/Current electricity/text/Resistance_networks/index.html?PHPSESSID=5b0029c25a5894099c6df916f68d95ac

(b) By symmetry

i1 = 5i2

i3 = 14 i2

i = 24 i2

Therefore:

Total resistance (R) = 7/12 r