Trying to transpose:

Rt = (R1R2/R1+R2) for R2

seem to keep going around in circles. Once I get to: (R1Rt+R2Rt)/R1=R2 I get stuck. Just can't seem to isolate R2 from l.h.s.

Problem relates to establishing the value of R2 (resistance, value to be calculated) to be added to R1(given value) in parallel to give Rt (given value)

Anybody help please

From your stuck point...

• multiply both sides by R1

• deduct R2Rt from both sides

• factor out the R2 from the RHS

• divide both sides by (R1 - Rt)

Many thanks. I can now make progress.
I just couldn't see wood for trees. Thanks again.