# probability

Printable View

• October 28th 2008, 10:28 AM
math_lete
probability
Hi can anybody help with this probability question?
i have 8 purple socks 4 green socks and 4 red socks in a drawer. two socks are chosen without replacement. What is the probability that a pair the same colour is chosen?
thanks
• October 28th 2008, 11:21 AM
Soroban
Hello, math_lete!

There are at least two ways to approach this problem . . .

Quote:

There are 8 purple socks 4 green socks and 4 red socks in a drawer.
Two socks are chosen without replacement.
What is the probability that a pair the same colour is chosen?

$\begin{array}{ccccc}P(\text{2 purple}) &=& \frac{8}{16}\cdot\frac{7}{15} &=& \frac{14}{60} \\ \\[-4mm]
P(\text{2 green}) &=& \frac{4}{16}\cdot\frac{3}{15} &=& \frac{3}{60} \\ \\[-4mm]
P(\text{2 red}) &=&\frac{4}{16}\cdot\frac{3}{15} &=& \frac{3}{60} \end{array}$

Therefore: . $P(\text{2 purple or 2 green or 2 red}) \;=\;\frac{14}{60} + \frac{3}{60} + \frac{3}{60} \;=\;\frac{20}{30} \;=\;\frac{1}{3}$

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

There are: ${16\choose2} = 120$ possible pairs.

. . There are: ${8\choose2} = 28$ ways to get two purple socks.

. . There are: ${4\choose2} = 6$ ways to get two green socks.

. . There are: ${4\choose2} = 6$ ways to get two red socks.

Hence, there are: . $28 + 6 + 6 \:=\:40$ ways to get a matching pair.

Therefore: . $P(\text{matching color}) \;=\;\frac{40}{120} \;=\;\frac{1}{3}$