What would be the effective resistance in this circuit?

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

• 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. – user1890202 Feb 15 '17 at 13:26
• Look for Delta-Y-transformation. – Curd Feb 15 '17 at 13:26
• @Curd not necessary in this case (think balanced Wheatstone bridge) – JIm Dearden Feb 15 '17 at 13:27
• @JIm Dearden: yes, you are right (because R3/R6=R2/R5). The center resistor R4 can be ignored. – Curd Feb 15 '17 at 13:29 