Basics physics question about calculating resistance
If you have a circuit with a 50 V battery and a single 50 ohm resistor there would be a current of 1A flowing through that resistance in accordance with Ohm's Law (V=IR). Now add in a 20 ohm resistor in parallel with this 50 ohm resistor. The current passing through the original 50 ohm resistor will be the same due to Ohm's Law (V=IR): The voltage drop across the resistor is the same (50V) and the resistance is the same (50 ohms) so you would have the same current. However, now the electrons can also flow through the other resistor so the overall current is larger. If you now zoom out and look at the overall circuit you have the same voltage as before but a bigger total current so the total (equivalent) resistance would have to be smaller (since from V=IR for V to be the same if I gets bigger R must get smaller). And this total resistance is given by that equation.