P(A or B) = P(A) + P(B) – P(A and B)
P(A) is the probability that event A will occur.
P(B) is the probability that event B will occur.
P(A or B) gives us the union; i.e. the probability that at least one of the two events will occur.
P(A and B) gives us the intersection; i.e. the probability that both events will occur.
Now, how do you find the value of P(A and B)? The value of P(A and B) depends on the relation between event A and event B. Let’s discuss three cases:
1) A and B are independent events
If A and B are independent events such as “the teacher will give math homework,” and “the temperature will exceed 30 degrees celsius,” the probability that both will occur is the product of their individual probabilities.
Say, P(A) = P(the teacher will give math homework) = 0.4
P(B) = P(the temperature will exceed 30 degrees celsius) = 0.3
P(A and B will occur) = 0.4 * 0.3 = 0.12
2) A and B are mutually exclusive events
If A and B are mutually exclusive events, this means they are events that cannot take place at the same time, such as “flipping a coin and getting heads” and “flipping a coin and getting tails.” You cannot get both heads and tails at the same time when you flip a coin. Similarly, “It will rain today” and “It will not rain today” are mutually exclusive events – only one of the two will happen.
In these cases, P(A and B will occur) = 0
3) A and B are related in some other way
Events A and B could be related but not in either of the two ways discussed above – “The stock market will rise by 100 points” and “Stock S will rise by 10 points” could be two related events, but are not independent or mutually exclusive. Here, the probability that both occur would need to be given to you. What we can find here is the range in which this probability must lie.
Maximum value of P(A and B):
The maximum value of P(A and B) is the lower of the two probabilities, P(A) and P(B).
Say P(A) = 0.4 and P(B) = 0.7
The maximum probability of intersection can be 0.4 because P(A) = 0.4. If probability of one event is 0.4, probability of both occurring can certainly not be more than 0.4.
Minimum value of P(A and B):
To find the minimum value of P(A and B), consider that any probability cannot exceed 1, so the maximum P(A or B) is 1.
Remember, P(A or B) = P(A) + P(B) – P(A and B)
1 = 0.4 + 0.7 – P(A and B)
P(A and B) = 0.1 (at least)
Therefore, the actual value of P(A and B) will lie somewhere between 0.1 and 0.4 (both inclusive).
Now let’s take a look at a GMAT question using these fundamentals:
There is a 10% chance that Tigers will not win at all during the whole season. There is a 20% chance that Federer will not play at all in the whole season. What is the greatest possible probability that the Tigers will win and Federer will play during the season?
Let’s review what we are given.
P(Tigers will not win at all) = 0.1
P(Tigers will win) = 1 – 0.1 = 0.9
P(Federer will not play at all) = 0.2
P(Federer will play) = 1 – 0.2 = 0.8
Do we know the relation between the two events “Tigers will win” (A) and “Federer will play” (B)? No. They are not mutually exclusive and we do not know whether they are independent.
If they are independent, then the P(A and B) = 0.9 * 0.8 = 0.72
If the relation between the two events is unknown, then the maximum value of P(A and B) will be 0.8 because P(B), the lesser of the two given probabilities, is 0.8.
Since 0.8, or 80%, is the greater value, the greatest possibility that the Tigers will win and Federer will play during the season is 80%. Therefore, our answer is E.
Karishma, a Computer Engineer with a keen interest in alternative Mathematical approaches, has mentored students in the continents of Asia, Europe and North America. She teaches the GMAT for Veritas Prep and regularly participates in content development projects such as this blog!