In order to find the total Resistance, lets break down how resistances are totaled
When in series, resistances add
When in parallel, resistances' inversed are added and then inversed again
So when you add the two resistors on the top, R1+2 = 5 ohms
To add the final resistor, since the bottom one is in parallel, we have to do
1/5 + 1/5 = 1/R
RTotal = 2.5 ohms
