The diode has resistance RD when it is conducting.Right, the diode has no resistance. We only have the resistance of the RD.
You already had R from the first case. You were looking for RD in the second case. But the same math applies, and you have in fact found RD.First graph has 40 Ω. Second has 20 Ω as net resistance. And since we have 1 / 20 = 1 / 40 - 1 / R so R = 40 Ω. Like this?