Carcass wrote:
The probability of winning game A is X and the probability of winning game B is Y. When playing a single round of each of the two games, what is the probability of winning exactly once?
A. X-YX
B. XY
C. X+Y
D. X+Y-2XY
E. X^2Y^2-XY^2-X^2Y
Key concept: P(Event A happening) = 1 - P(Event A not happening)If P(win game A) = X, then P(lose game A) = 1 - X
If P(win game B) = Y, then P(lose game B) = 1 - Y
P(win exactly once) = P(win game A
AND lose game B
OR lose game A
AND win game B)
= P(win game A)
x P(lose game B)
+ P(lose game A)
x P(win game B)
= X
x (1 - Y)
+ (1 - X)
x Y
= X - XY
+ Y - XY
= X + Y - 2XY
Answer: D