1. The problem statement, all variables and given/known data What is the value of the unknown resistor in the circuit shown if the equivalent resistance for the circuit is 21.50 ohms. 2. Relevant equations Series= R1+R2+R3... Parllel= (1/R1)+(1/R2)+(1/R3)... 3. The attempt at a solution First off, I'm sorry for the horrendous drawing I did, but its better than nothing i guess. I have done several of these problems and I am STILL stuck on every single one of them. What I did was I drew several different possibilities of series and parallel circuits, but none of them came anywhere near the 21.50 ohms. First I drew a pic, that went like this: (series) 7.5 and 9.1 then parallel 6.81 and ? then series again 9.1 and 3.6. However, just adding the series circuits together I came up with 29.3, and that was before the Parallel circuit. Then I tried leaving the very first circuits (the first 9.1 and 7.5) and series, and making everything else parallel: 1/9.1= .11 1/3.6= .29 1/6.81= .15 .11+.29+.15= .55 1/.55= 1.82 7.5+9.1= 16.6 16.6+1.82= 18.42 Im fairly certain that since the unknown factor would be prallel you can't just subtract 18.42 from 21.50 How do I find this? And, am I even close to guessing the correct parallel and series combination? I'm clueless, an help is appreciated.