Answer:
The missing probability p = 0.22.
Step-by-step explanation:
A fully charged cell is composed of nickelic hydroxide in a NiCd battery.
Nickel is known to be such an element that it has multiple oxidation states.
The proportions of the states are given as following. Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
          X                  p(X)
          0                   p
          2                   0.23
          3                   0.35
          4                   0.2
The total probability of an event, which in this case, is the event that Nickel has oxidation states of either 0, 2 , 3 or 4, is always equal to 1.
Therefore we can write p + 0.23 + 0.35 + 0.2 Â = 1
Therefore  p = 1 - (0.23 + 0.35 + 0.2) = 1 - 0.78 = 0.22
The missing probability p = 0.22.