How would one go about calculating the resistance between points A and B on the following circuit:

enter image description here

This is for Higher Secondary level. The person who needs to solve this understands these basic rules:

-> Combined resistance of two resistors in series is the sum of their individual resistances.

-> Resistance of two resistors in parallel is the inverse of (1/R1 + 1/R2)

-> Ohm's law, V=IR

Furthermore, I know how to calculate the combined resistance if either one of R2 or R6 was absent (it would be 100 + (1/100+1/200)^-1), but their combined presence makes this confusing.

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    \$\begingroup\$ Stackexchange is not really meant for homework but I'll give you a hint: the voltage across R4 is 0V ie no current flows through it. \$\endgroup\$ – user1890202 Feb 15 '17 at 13:26
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    \$\begingroup\$ Look for Delta-Y-transformation. \$\endgroup\$ – Curd Feb 15 '17 at 13:26
  • \$\begingroup\$ @Curd not necessary in this case (think balanced Wheatstone bridge) \$\endgroup\$ – JIm Dearden Feb 15 '17 at 13:27
  • \$\begingroup\$ @JIm Dearden: yes, you are right (because R3/R6=R2/R5). The center resistor R4 can be ignored. \$\endgroup\$ – Curd Feb 15 '17 at 13:29

This is a classic tricky question. If you were required to know loop analysis the solution would be easy. Since you don't, you need to think about this a bit differently. Start by redrawing the circuit


simulate this circuit – Schematic created using CircuitLab

Now remove R4, and think about the voltages at C and D. Will any current flow through R4 if you connect it back into the circuit? Does this mean you can simplify the circuit? Do you now see how to calculate the resistance?

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