Bdtyk

The question:

We are given 6 different ingredients. 2 types of flours, 2 types of sugars, and 2 identical eggs.

$$F_1,F_2,S_1,S_2,E,E$$

We choose 3 items at random, we mix them, and throw them into the oven.

A) What is the sample space of the question

B) What is the probability we make a cake (flour + sugar + egg)

My attempt:

A) This would just be all permutations of possible choices, the size of the sample space would be 6 choose 3 (although I don't think this is correct because the eggs are identical)

How would I solve question B?

In an attempt to make the problem easier to calculate, we may choose to take one of the two identical eggs and mark it with a foodsafe marker of some sort. Now, the eggs are in fact distinguishable. In this way we would have $binom{6}{3}$ outcomes in our sample space. We generally choose to do this because we have that each of these $binom{6}{3}=20$ outcomes are equally likely to occur (given the most common interpretation of the problem).

If you insist that you want to look at the sample space as being where the eggs are treated as identical, you will have overcounted each outcome where exactly one egg appears twice each when we wanted to count them only once. There are $binom{4}{2}=6$ such outcomes being overcounted. This drops our sample space down to size $binom{6}{3}-binom{4}{2}=14$. This is not a useful sample space for performing calculations with however since the outcomes are not equally likely to occur. It is far more likely to have an outcome of $(F_1, S_1, E)$ than $(F_1, E, E)$ (specifically it is twice as likely to occur).

Remember that if we want to use $Pr(E) = dfrac{|E|}{|S|}$, we require that each outcome in the sample space be equally likely to occur. There are only two outcomes to playing the lottery, you either win or you lose. The probability of winning the lottery is nowhere close to $frac{1}{2}$ however despite this.

Making the proposed simplification to the problem where we mark one of the eggs ahead of time, we count how many outcomes will actually result in one of each type of ingredient being used. There are $2$ ways to select which flour is used, $2$ ways to select the sugar, and $2$ ways to select the egg. Multiplying the number of options for each gives the total number of arrangements of one of each type as per the rule of product. This yields $2times 2times 2=8$ ways to select one from each ingredient type.

Dividing by the size of the sample space (remembering that this sample space was chosen specifically because it is an equiprobable sample space that fully describes the event we are interested in), gives us the probability:

$$frac{8}{20}=0.4$$

The probability you make a cake is equal to the number of three-ingredient combinations in the space that form a cake (flour + sugar + egg), divided by the total number of three-ingredient combinations in the space. The total number of three-ingredient combinations is 20 (= 6 choose 3), and because you have two possibilities for each of flour, sugar, and egg, the number of three-ingredient-combinations that make a cake is 2*2*2 = 8. So the probability of making a cake is 8 / 20, or 2/5 = 40%.