Conditional probability question - not sure how to go about this? It may be the wording of the question, but I'm not sure how to go about doing this. This is not a homework question I just want to know if my answer is correct.
*Please note the wording is confusing, but please interpret however you understand.
90% of candidates to a job position can code both in Python and Perl. 70% of these candidates can code in Python and 50% can code in Perl. What is the probability that a candidate can code in Perl knowing that he can code in Python?'
The way I interpreted this was that the probability that a candidate can code in both is 20% (is this assumption correct?)
In the end I found that the answer is 2/7.
If someone can verify this I would be very thankful, as I'm not sure I went about it the right way!
 A: I assume that by writing

90% of candidates to a job position can code both in Python and Perl

they actually meant "can code either in Python or Perl". This is consistent with the rest of the question, as they divide this quantity further into Perl-coders and Python-coders.
In such questions, it helps to denote the variables. Let's denote by $A$ the event that a person knows Python and by $B$ the event that a person knows Perl. Now, in the question we are given the following quantities:
$$
P\left(A \cup B\right) = 0.9 \\
P\left(A \mid A \cup B\right) = 0.7 \\
P\left(B \mid A \cup B\right) = 0.5
$$
And our final goal is to calculate $P\left(A\mid B\right)$.
Now, this question consists of multiple steps. You don't need to assume anything since you have enough data. You need to first calculate $P\left(A\right)$, $P\left(B\right)$, then calculate $P\left(A \cap B\right)$, and only then you can calculate $P\left(A \mid B\right)$. The following identifies might be helpful:
$$
P\left(A \cup B\right) = P\left(A\right) + P\left(B\right) - P\left(A \cap B\right) \\
P\left(A \mid B\right) = \frac{P\left(A \cap B\right)}{P\left( B\right)}
$$
The final result should be $P\left(A \mid B\right) = 0.4$
