Equivalent resistance in this mosfet circuit
Redrawing the ckt like this may ring some bells.

For example the first ckt:







simulate this circuit &ndash; Schematic created using CircuitLab


If the mosfet is on, substitute it by its on-resistance \$R_{on}\$
If the mosfet is off, substitute it by open ckt.
Find the net resistance that comes across A and B in parallel to capacitor