The equation is
So, it is actually “1 divided by (1/20 + 1/10)” which is then “1 divided by .15”
That equation is a mathematical representation of what the equivalent resistance would be if all loads that are in parallel with each other in a particular configuration were somehow combined to be just a single load. In other words, it shows the net affect on the power source (in terms of resistance) of multiple loads that are in parallel.
When loads are in series with each other, you simply add the resistances to get the same kind of thing (equivalent, or “total”, resistance of the circuit).
Does that help?