Video Transcript
Suppose that 𝐴 and 𝐵 are two
events. Given that the probability of 𝐵 is
0.06, the probability of 𝐴 union 𝐵 is 0.09, and 𝐵 is a subset of 𝐴, determine the
probability of 𝐴.
We will begin this question by
drawing a Venn diagram to model the situation. We are told that 𝐵 is a subset of
𝐴, which means that every element in 𝐵 is also contained in 𝐴. The question tells us that the
probability of 𝐵 occurring is 0.06. And it also says that the
probability of 𝐴 union 𝐵 is 0.09. This is the probability that 𝐴
occurs or 𝐵 occurs or they both occur.
To determine the probability of 𝐴,
we recall one of our probability rules, which should be clear from the Venn
diagram. If 𝐵 is a subset of 𝐴, then the
probability of 𝐴 is equal to the probability of 𝐴 union 𝐵. The probability of 𝐴 is therefore
equal to 0.09.
Whilst it is not required in this
question, we could complete the Venn diagram as follows. Firstly, the probability of only 𝐴
occurring, written 𝑃 of 𝐴 minus 𝐵, is equal to 0.09 minus 0.06, which is equal to
0.03. The probability of neither 𝐴 nor 𝐵
occurring is one minus the probability of their union, in this case one minus
0.09. This is equal to 0.91. We can check this by ensuring that
all the values in the Venn diagram sum to one.
