I have added an attachment of the circuit for clarity. I initially began by assuming that the 13 ohm and 24 ohm resistors were in parallel and I got the Req for them. Then since the top resistor seemed to be in series with them I added its value. This is wrong of course, the correct sequence of steps is: Treat the top resistor and the 13 ohm resistor as as if they are in series. Then with the attained Req add the 24 ohm resistor using the correct math for parallel resistors. Where did my thinking originally go wrong? My guess is that it has something to do with some sort of "order of operations" for circuits. Can that be clarified please?