well since no one answered you ill try my best to give you some direction.
I couldnt find the actual answer, but I'm fairly certain I know how to do it.
start of with a table
I set X as 0-3, because thats how many total possible choices there are.
The auditor, can either find no errors, 1 error, 2 errors or 3 errors.
so now you just have to multiply the total # of way to get any of the errors, times the probability of getting an error.
All the values for xp(x) should add up to one. If they dont, you did something wrong.