WEBVTT
00:00:00.560 --> 00:00:04.120
A class contains six boys and 21 girls.
00:00:04.800 --> 00:00:08.720
If a pupil is selected at random, what is the probability they are a girl?
00:00:10.040 --> 00:00:23.640
The probability of an event happening can be written as a fraction where the numerator, or top number, is the number of successful outcomes and the denominator, or bottom number, is the number of possible outcomes.
00:00:24.200 --> 00:00:34.600
In this case, the total number of pupils is 27 as six plus 21 equals 27.
00:00:35.200 --> 00:00:39.720
Therefore, the number of possible outcomes are also 27.
00:00:40.400 --> 00:00:43.760
We want to find the probability that the selected pupil is a girl.
00:00:44.560 --> 00:00:48.080
Well in this question, there are 21 girls.
00:00:48.640 --> 00:00:52.400
Therefore, the number of successful outcomes is 21.
00:00:53.000 --> 00:01:04.200
We can therefore say that the probability of selecting a girl at random is twenty-one twenty-sevenths or 21 out of 27.
00:01:05.120 --> 00:01:10.760
This fraction can be simplified or cancelled by dividing the numerator and denominator by three.
00:01:11.480 --> 00:01:14.880
21 divided by three is equal to seven.
00:01:15.560 --> 00:01:18.920
And 27 divided by three is equal to nine.
00:01:19.640 --> 00:01:32.480
This means that the probability of selecting a girl at random from a class containing six boys and 21 girls is seven out of nine or seven-ninths.