This is to calculate the usage of two resistors in parallel. We get the total resistance from the two resistors used by way of the following formula:
RTotal = 1 ÷ ((1÷R1) + (1÷R2) + (1÷Rn))
For a shortcut, if you have two resistors that share the same value, just divide that value by two.
When using resistors in series, you just add the values of each one in line. R1+R2+R3 ... and so on.
When creating, fixing or modifying equipment, parallel use of resistors is useful to come up with values that are lower to what you have on hand. In series, to get to a higher resistance value when your single resistor is too low.