 Two resistors in parellel

 
When two (or more) resistances are in parellel, then the resitence provided by the two together is given by reciprocal addition, i.e. 1/(total resistence) = (1/resistance1) + (1/resistance2).
 
The addition of reciprocals is a common solution for problems, aand this calc may be used to solve other similar relationships. For larger numbers of resistors, this calc may be used iteratively.  