RESISTANCES IN PARALLEL

When resistances are placed in parallel, they behave differently than they do in series. In general, if you have a resistor of a certain value and you place other resistors in parallel with it, the overall resistance decreases. Mathematically, the rule is straightforward, but it can get a little messy.
One way to evaluate resistances in parallel is to consider them as conductances instead. Conductance is measured in units called siemens, sometimes symbolized S. (The word siemens serves both in the singular and the plural sense). In older documents, the word mho (ohm spelled backwards) is used instead. In parallel, conductances add up in the same way as resistances add in series. If you change all the ohmic values to siemens, you can add these figures up and convert the final answer back to ohms.
The symbol for conductance is G. Conductance in siemens is the reciprocal of resistance in ohms. This can be expressed neatly in the following two formulas. It is assumed that neither R nor G is ever equal to zero:
G = 1/R
R = 1/G