# Law of Total Probability Examples



The law of total probability is explained and used to solve examples including detailed explanations.

## Explanation of the Law of Total Probability

Let the sample space $S$ be partioned into $n$ mutually exclusive events $E_1$, $E_2$, $E_3$ ... $E_n$ and collectively exhaustive (covering the entire sample space $S$ ). Note that
$A = (A \cap E_1) \cup (A \cap E_2) \cup (A \cap E_3) ... \cup (A \cap E_n)$
and since the events $E_i$ are all mutually exclusive, then $(A \cap E_i)$ are also all mutually exclusive and therefore we can use the addition rule of probabilities to write
$P(A) = P(A \cap E_1) + P(A \cap E_2) + P(A \cap E_3) ... P(A \cap E_n)$
Using the definition of conditional probabilities we write
$P(A \cap E_n) = P(A | E_n) P(E_n)$,
and rewrite the law of total probability as
$P(A) = P(A | E_1) P(E_1) + P(A | E_2) P(E_2) + P(A | E_3) P(E_3) ... P(A | E_n) P(E_n)$ $= \sum_{i=1}^{n} P(A | E_i) P(E_i)$ Note that because $E_1$, $E_2$, $E_3$ ... $E_n$ are collectively exhaustive, we have
$\sum_{i=1}^{n} P(E_i) = 1$

## The Law of Total Probability Examples with Detailed Solutions

We start with a simple example that may be solved in two different ways and one of them is using the the Law of Total Probability.

Example 1
We have three similar bags B1, B2 and B3 containing 4 balls each. B1 contains 2 red and 2 blue balls, B2 contains 3 red and 1 blue balls and B3 contains 1 red and 3 blue balls. One bag is selected at random and a ball is selected at random from that bag.
What is the probability that the ball is red?

Solution to Example 1
We start by a diagram that explains what is given. We present T two methods to solve the above question:
Method 1: Use classical method of calculating probabilities
In all three bags there is a total of 12 balls 6 of which are red balls. Let event $A$ be that of selecting a red ball. Hence the probability that the ball selected is red is given by
$P(A) = 6/12 = 1/2$
Method 2: Use law of total probability
Let $E_1$, $E_2$ and $E_3$ be the event of selectiong bag $B_1$, $B_2$ and $B_3$ respectively and $A$ be the event of selecting a red ball.
$P(A) = P(A | E_1) P(E_1) + P(A | E_2) P(E_2) + P(A | E_3) P(E_3)$
The bags are similar and are therefore equally likely to be selected. The probabilities of selecting bags $B_1$, $B_2$ or $B_3$ at random are given by
$P(E_1) = 1/3$ (one bag B1 out of 3 bags)
$P(E_2) = 1/3$ (one bag B2 out of 3 bags)
$P(E_3) = 1/3$ (one bag B3 out of 3 bags)
The conditional probabilities of selecting a red ball given that it is in bag $B_1$, $B_2$ and $B_3$ are given by
$P(A | E_1) = 2/4 = 1/2$ (2 red balls out of a total of 4 balls in B1)
$P(A | E_2) = 3/4$ (3 red balls out of a total of 4 balls in B2)
$P(A | E_3) = 1/4$ (1 red balls out of a total of 4 balls in B3)
We now use the law of total probability
$P(A) = (1/2)(1/3) + (3/4)(1/3) + (1/4)(1/3) = 1/2$

Note that the law of total probability will be used in situations where classical methods of calculating probabilities cannot be used as we will see in the examples below.

Example 2
Studenst in a math class where 40% are males and 60% are females took a test. 50% of the males and 70% of the females passed the test. What percent of students passed the test?

Solution to Example 2
We start by drawing a diagram with the whole class inclusing the males and females groups.
Let events $E_1$ "be a male" and $E_2$ "be a female", and event $A$ "passed the test". The law of total probability gives
$P(A) = P(A | E_1) P(E_1) + P(A | E_2) P(E_2)$
$= 50\% \times 40\% + 70\% \times 60\% = 62\%$

Example 3
Three factories produce the same tool and supply it to the market. Factory A produces 30% of the tools for the market and the remaining 70% of the tools are produced in factories B and C. 98% of the tools produced in factory A, 95% of the tools produced in factory B and 97% of the tools produced in factory C are not defective.
What percent of tools should be produced by factories B and C so that a tool picked at random in the market will have a probability of being non defective equal to 96%?

Solution to Example 3
We start by drawing a diagram of all three factories.
Let $A$, $B$ and $C$ be the events of tools produced by factories A, B and C and $ND$ be the event "non defective".
$x$ and $y$ are the percentages of tools to be produced by companies B and C respectively. Using the above diagram, we can write
$P(A) = 30\%$ , $P(B) = x$ , $P(C) = y$
The three factories supply the whole market; hence
$30\% + x + y = 100\%$ which gives $y = 70\% - x$
Use the law of total probability to write the equation
$P(ND) = P(ND|A) P(A) + P(ND|B) P(B) + P(ND|C) P(C)$
$= 98\% \times 30\% + 95\% \times x + 97\% \times (70\% - x) = 96\%$
Solve for $x$
Company B produces: $x = 0.65 = 65\%$
Company C produces: $y =70\% - 65\% = 5\%$

Example 4
5% of a population have a flu and the remaining 95% do not have this flu.
A test is used to detect the flu and this test is positive in 95% of people with a flu and is also (falsely) positive in 1% of the people with no flu.
If a person from this population is randomly selected and tested, what is the probability that the test is positive?

Solution to Example 4
Use the total probability theorem to find the percentage as follows:
$5\% \times 95\% + 95\% \times 1\% = 5.7\%$