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First of all, let me notice, that your questions (a), (b) and (c) are posed INCORRECTLY.
My EDITING to your questions is as follows.
(a) What is the probability that a selected employee knows both languages?
(b) What is the probability that a selected employee knows C/C++ but not Java?
(c) What is the probability that a selected employee knows only one of the two languages?
The rest of questions (d), (e) and (f) are correct.
Solution
We are given
P(C) = 0.7, P(J) = 0.6, and P(C U J) = 0.9.
Then P(C ∩ J) = P(C) + P(J) - P(C U J) = 0.7 + 0.6 - 0.9 = 0.4.
Now I am in position to answer all questions, one after another.
(a) This probability is P(C ∩ J) = 0.4.
(b) This set is C \ (C ∩ J); therefore,
this probability is equal to P(C) - P(C ∩ J) = 0.7 - 0.4 = 0.3.
(c) This probability is equal to P(C U J) - P(C ∩ J) = 0.9 - 0.4 = 0.5.
(d) This probability is P(C ∩ J)/P(J) = = .
(e) This probability is P(C ∩ J)/P(C) = = .
(f) To answer first part, compare P(C)*P(J) with P(C ∩ J).
They are different; therefore, the answer is "NO".
To answer the second part, compare P(C ∩ J) with 0 (zero, ZERO).
They are different; therefore, the answer is "NO".