I have included a link to the online calculator for parallel circuits.This will calculate the total resistances of the circuit that you input.(note: na is the default value in case you have to delete one.)

https://appsnap.us/electro/resistor.html

series circuits

formula for series of resistors is the sum of all of the resistors on the series circuit.

total resistance=20+20+40 or 80 ohms.