Question

The probability of event A is x, and the probability of event B is y. If the two events are independent, which of these conditions must be true?
a. P(B|A) = y
b. P(A|B) = y
c. P(B|A) = x
d. P(A and B) = x + y

e. P(A and B) = x/y

P(A)

Answer 1

If two events are independent, then

$Pr(A\cap B)=Pr(A)\cdot Pr(B)$.

Use formulas for conditional probabilities:

$Pr(A|B)=\dfrac{Pr(A\cap B)}{Pr(B)},\\ \\ Pr(B|A)=\dfrac{Pr(A\cap B)}{Pr(A)}$.

For independent events these formulas will be:

$Pr(A|B)=\dfrac{Pr(A\cap B)}{Pr(B)}=\dfrac{Pr(A)\cdot Pr(B)}{Pr(B)}=Pr(A),\\ \\ Pr(B|A)=\dfrac{Pr(A\cap B)}{Pr(A)}=\dfrac{Pr(A)\cdot Pr(B)}{Pr(A)}=Pr(B)$.

Now in your case $Pr(A)=x,\ Pr(B)=y$ and $Pr(A|B)=x,\ Pr(B|A)=y, Pr(A\cap B)=x\cdot y$.

This shows that the only correct choice is A.

Answer 2

The right answer for the question that is being asked and shown above is that: "d. P(A and B) = x + y." The probability of event A is x, and the probability of event B is y. If the two events are independent, the condition must be true is this d. P(A and B) = x + y