When two or more resistors are connected in series, the equivalent resistance (Req) is the sum of the individual resistances:
R1 + R2 + R3 ... = Req
For example, if three 100Ω resistors are connected in series, the equivalent resistance is 100Ω+100Ω+100Ω=300Ω.
The equivalent resistance of a circuit can also be calculated when resistors are connected in parallel. This is done by taking the reciprocal of the sum of the reciprocals of each individual resistor's value. The mathematical formula for this is:
1/Req = 1/R1 + 1/R2 + 1/R3 ...
For example, if three 100Ω resistors are connected in parallel, the equivalent resistance would be 1/(1/100Ω+1/100Ω+1/100Ω)=0.33 Ω