The potential differences across the resistance, capacitance, and...

${P}_{ac} = {V}_{rms}{I}_{rms}\cos \left(ϕ\right)$,

where, $\cos \left(ϕ\right)$ is the power factor.

$cosϕ = \dfrac{R}{z} = \dfrac{{V}_{R}}{V} = \dfrac{80}{\sqrt{{80}^{2} + {\left(100 - 40\right)}^{2}}}$

$= \dfrac{80}{100} = 0.8$.

where, $Z$ is the impedence of the circuit.