What happens to the resistance of the circuit when you add resistances in parallel?

Adding a resistance in parallel to another resistance will reduce the equivalent resistance.

Parallel Equation:

R1 = 10Ω

R2 = 20Ω

Req = R1R2/(R1+R2) = (1/R1+1/R2)-1

Req = 10•20/(10+20) = 6.67Ω