In this case, the question is: What are the odds of a parahax event where the defending pokemon is paralyzed by Thunderbolt then is unable to attack due to paralysis happening over the course of n turns?
It would be something like
Turn 1: 0.025
Turn 2: 0.025 + ((0.075) * 0.25)
Turn 3: 0.025 + ((0.075) * 0.25) + (((0.075) * 0.75) * 0.25)
Turn 4: 0.025 + ((0.075) * 0.25) + (((0.075) * 0.75) * 0.25) + ((((0.075) * 0.75) * 0.75) * 0.25)
So for turn 1 we have the chance of paralysis (0.1 or 10%) times the chance of full paralysis (0.25 or 25%) to give us 0.025.
For turn 2 we have the same chance of paralysis, plus the chance that on turn 1 you were paralyzed but not fp (0.1 - 0.025 or 0.075) times the chance of fp (0.25).
For turn 3 we have the chance of paralysis + fp (0.025) plus the chance of a paralysis turn 2 and no full paralysis on turn 2, times the fp chance (1 - 0.025 * 0.25), PLUS the chance of a paralysis on turn 1 and no fp on turns 1 or 2, times the fp chance (((0.075) * 0.75) * 0.25).
For turn 4 we have the chance of paralysis + fp (0.025) plus the chance of a paralysis turn 3 and no full paralysis on turn 3, times the fp chance (1 - 0.025 * 0.25), plus the chance of a paralysis on turn 2 and no fp on turns 2 or 3, times the fp chance (((0.075) * 0.75)) * 0.25), PLUS the chance of a paralysis on turn 1 and no fp on turns 1, 2 or 3 ((((0.075) * 0.75) * 0.75) * 0.25).
Anyway, the totals are as follows:
Turn 1: 0.025 or 2.5%
Turn 2: 0.04375 or 4.4%
Turn 3: 0.0578125 or 5.8%
Turn 4: 0.068359375 or 6.8%
Feel free to do the next few numbers following that pattern but I can't be bothered.