Well first of all you look at your original set of possible outcomes (reminder NN ,CN, NC, CC all equally likely, SUM MUST BE 100%) and you modify it to match the new conditions, namely 2 crits
leaving us with only (... ... ... CC, all equally likely, SUM MUST BE 100%).
There is only one possible outcome matching all given criteria and if only one outcome is possible the propability of that outcome is... well 100%
Now we do the exact same steps again, only this time with a different condition> and at least one of them was a crit
Again we have 4 possible outcomes (NN ,CN, NC, CC all equally likely, SUM MUST BE 100%) and again we sort out the outcomes that match the given condition (at least one crit), but this time we are left with 3 possible outcomes
(..., CN, NC, CC all equally likely, SUM MUST BE 100%). 3 possible outcomes, all equally likely, you dont need a masters degree in mathematics to figure out that thats a chance of 1/3 or 33,333...%.
Alright almost there
We were asked for the propability of him having 2 crits.
take a look at the set of possible outcomes again and see if you can figure out which of them match this condition, then add up their propabilities.
33,33% CN --> Nope
33,33% NC --> Nope
33,33% CC --> yes
33,33% or exactly 1/3
there are 3 possible outcomes matching the condition "at least 1 crit", all equally likely (1/3) and within this set of 3 possible outcomes there is only one outcome matching the condition "2 crits", therefore
the propability of him having 2 crits is and will always be exactly 1/3
congratulations Anon !! you have now reached the mathematical understanding of a 5th grader