Could someone please help with the above question? Despite the advice already given https://forum.allaboutcircuits.com/threads/dc-series-parallel-worksheet.67244/ I am finding it difficult to arrive at the answer of 80.54 ohms.

The hint is to consider the 330 and 470 ohm resistors in parallel. However, if I start with the 100 and 220 resistors in parallel I get an equivalent resistance of 68.75 ohms, which seems to show I have already made a mistake.

There is no harm in providing me with a full explanation of the steps as I am pursuing this course at home for my interest only. Many thanks!

 

hi haydn,
Welcome to AAC.
Is this the circuit you are asking about.?
E

 

hi haydn,
Post your calculations so that we can see where you have gone wrong.
E

 

Hi eric,
Thanks very much for your reply. Yes, that's the circuit. As far as my calculations are concerned, I have done so many permutations! After seeing bountyhunter's hint, I suppose my basic assumption has been parallel pair in parallel with parallel pair (as opposed to parallel pair in series with parallel pair, which seems to be correct for the similar Q.5, fig. 3), thus 220//100 (= 68.75) // 470//330 (= 193.875), so 68.75//193.875 = 50.75 ohms, wrong. After that, assuming 470//330 is a valid branch, then my next idea is some sort of series concept of the 220 or 100 ohm resistors, but I can not make that work either

Before I saw the bountyhunter's hint, the closest, I'm sure uninformed, approach I had was to take 220-330//100 = 84.61 ohms, with the 470 ohm resistor unaccounted for, as a sort of buffer (!) between the parallel configuration.

I note that 100//470 ohms combines to 82.46 ohms.

For the purposes of this exercise, I assume that point A represents a voltage or current source, but at this stage of my engagement with the AAC textbook I don't know if polarity or current flow subtleties are important to this particular problem.

 

hi,
This diagram should make the sequence clear, ask if you have a problem.

E