The inverse of resistance (1/Ω) is conductance.

Conductances are added in parallel.

1/10Ω + 1/15Ω = 3/30 + 2/30 = 5/30 = 1/6

Resistance = inverse 1/6 = 6Ω

Resistances are added in series.

20Ω + 6Ω = 26Ω

This applies to:

Thermal Conductance - Insulation

Vapour permeability - Construction

Airflow in ducting - Ventilation

Water flow in pipes - Plumbing