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We are just in the beginning phases of learning how to simplify circuits by finding equivalent resistance. I've gotten every problem right except for this one. The answer says its 2000 ohms, but I keep getting 1200 Ohms. My method was by combining the 480 and 320 Ohm resistors, and then doing the math to combine them with the 1200 ohm resistor. Then added 720, as it was in series at that point.enter image description here

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  • \$\begingroup\$ Not sure why "the answer says its 2000 ohms" is this a problem split to three parts (a, b and c)? \$\endgroup\$ – Iancovici Feb 8 '16 at 14:23
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    \$\begingroup\$ The question is a bit vague for me.What does the problem ask you to do? \$\endgroup\$ – Daniel Tork Feb 8 '16 at 15:28
  • \$\begingroup\$ You've given us the schematic and told us the answer, but we still don't know what the actual question is. For all we know it might NOT be "what is the resistance measured between A and B?"... \$\endgroup\$ – Brian Drummond Feb 8 '16 at 15:44
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    \$\begingroup\$ The question might be, 'what is the resistor that must be added between nodes a and b so that the total resistance between those nodes becomes 251.7 ohms'! \$\endgroup\$ – Neil_UK Feb 8 '16 at 16:16
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Any pair of resistors that share two nodes are in parallel.

  1. 720 ohms isn't in series with 1200, it's in parallel.

  2. Combination of 320 and 480, are in parallel with 720 and 1200 as well.

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320 and 480 are in series - the resistances add up to 800

720, 1200 and 800 are in parallel.

schematic

simulate this circuit – Schematic created using CircuitLab

The answer is wrong however, the combined resistance of parallel resistors is smaller than the smallest resistor, so it should be smaller than 720. The real answer should be 288.

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