### Counting Techniques for Probability Ratio using Combination

The probability ratio of an event is the likelihood of the chance that the event will occur as a result of a random experiment, and it can be found using combination. When the number of possible outcomes of a random experiment is infinite, the enumeration or counting of the sample space becomes tedious. In these situation we make use of what is known the counting technique principle for finding probability. Combinations will be used if the order of the events are not significant.

\(\require{color}\)

$$ \binom{n}{k} = \dfrac{n!}{(n-k)!k!} $$

Let’s take a look at the worked examples now.

#### Probability using Combination

A bag contains \(n\) pink cards, \(n\) yellow cards and \(n\) green cards. Three cards are drawn at random from the bag, one at a time, without replacement.

(a) Find the probability, \(P_s \), that the three cards are all the same colour.

It could be three pink cards or three yellow cards or three green cards out of \(3n\) cards.

\( \newcommand\ddfrac[2]{\frac{\displaystyle #1}{\displaystyle #2}} \)

The probability that the three cards are all pink is \(\displaystyle \ddfrac{\binom{n}{3}}{\binom{3n}{3}} \).

The probability that the three cards are all yellow is \(\displaystyle\ddfrac{\binom{n}{3}}{\binom{3n}{3}} \).

The probability that the three cards are all green is \(\displaystyle\ddfrac{\binom{n}{3}}{\binom{3n}{3}} \).

\( \therefore P_s = \displaystyle\ddfrac{\binom{n}{3}}{\binom{3n}{3}} \times 3 \)

(b) Find the probability, \(P_s \), that the three cards are all the different colour.

It should be one pink card from \(n\) pink cards, one yellow card from \(n\) yellow cards and one green card from \(n\) green cards.

\( \therefore P_d = \displaystyle\ddfrac{\binom{n}{1} \times {n\choose 1} \times {n\choose 1}}{\binom{3n}{3}} \)

(c) Find the probability, \(P_m \), that the two cards are of one colour and the third is of a different colour.

There are **PPY PPG YYP YYG GGP GGY**, so six ways.

\( \therefore P_d = \displaystyle\ddfrac{\binom{n}{2} \times {n\choose 1}}{\binom{3n}{3}} \times 6 \)

(d) If \(n\) is large, find the approximate probability ratio \(P_s : P_d : P_m \).

\( \begin{aligned} \displaystyle

&= \ddfrac{\binom{n}{3}}{\binom{3n}{3}} \times 3 : \ddfrac{\binom{n}{1} \times {n\choose 1} \times {n\choose 1}}{\binom{3n}{3}} : \ddfrac{\binom{n}{2} \times {n\choose 1}}{\binom{3n}{3}} \times 6 \\

&= \binom{n}{3} \times 3 : \binom{n}{1} \times {n\choose 1} \times {n\choose 1} : \binom{n}{2} \times {n\choose 1} \times 6 \\

&= \frac{n!}{(n-3)!3!} \times 3 : n \times n \times n : \frac{n!}{(n-2)!2!} \times n \times 6 &\color{green} \binom{n}{k} = \dfrac{n!}{(n-k)!k!} \\

&= \frac{n(n-1)(n-2)(n-3)!}{(n-3)! \times 6} \times 3 : n^3 : \frac{n(n-1)(n-2)!}{(n-2)! \times 2} \times 6n \\

&= \frac{n(n-1)(n-2)}{2} : n^3 : 3n^2(n-1) \\

&= \frac{1}{2}n^3 – \frac{3}{2}n^2 + n : n^3 : 3n^3 – 3n^2 \\

&= n^3 – 3n^2 + n : 2n^3 : 6n^3 – 6n^2 \\

\text{approximate ratio } &= \lim_{n\to\infty} n^3 – 3n^2 + n : \lim_{n\to\infty} 2n^3 : \lim_{n\to\infty} 6n^3 – 6n^2 \\

&= \lim_{n\to\infty} \bigg(\frac{n^3}{n^3} – \frac{3n^2}{n^3} + \frac{n}{n^3}\bigg) : \lim_{n\to\infty} \bigg(\frac{2n^3}{n^3}\bigg) : \lim_{n\to\infty} \bigg(\frac{6n^3}{n^3} – \frac{6n^2}{n^3}\bigg) \\

&= \lim_{n\to\infty} \bigg(1 – \frac{3}{n} + \frac{1}{n^2}\bigg) : \lim_{n\to\infty} 2 : \lim_{n\to\infty} \bigg( 6 – \frac{6}{n}\bigg) \\

&= 1 : 2 : 6 \\

\end{aligned} \\ \)