My brother and his wife and 2 kids — the residents of The Shire — have a rare birthday condition. Rowland is a Leo, Shannon is a Taurus. One of the boys, Roman, is a Leo; the other, a Taurus.
What are the odds of that?
Of what? It’s crucial to write out the problem first. Let’s talk this through and establish facts.
The are 12 zodiac signs. Every person has one and only one sign.
There are 2 pairs of people. The first pair consists of the sign Leo and the sign Taurus. The second pair consists of a Leo and a Taurus.
What are the odds that both Rowland and Roman are Leo?
1/12 * 1/12
What are the odds that both Shannon and John are Taurus?
1/12 * 1/12
(Notice how we didn’t look for the odds of Roman being a Leo BUT NOT a Taurus…
Or of John being a Taurus BUT NOT a Leo…
That would be redundant. The 1/12 handles it already. Being a Leo excludes Roman from being a Taurus. )
So the answer to the above problem is: 1/12 * 1/12 * 1/12 * 1/12
Those are VERY high odds. 4.74 e-05
But what if we look instead at the Kind of Arrangement rather than the specific signs? I think this is the question being asked:
The are 12 zodiac signs. Every person has one and only one sign.
There are 2 pairs of people. The first pair (parents) consists of 2 different signs. The second pair’s signs match up with the first pair.
Let’s take that one at a time:
1. What are the odds that 2 people would not have the same zodiac sign?
A: Set the sign of person 1 to a value, X, as a constant. (X in this case can be “Leo”). The constant is equal to 1, because there was a 100% chance person 1 would have a sign.
So, what are the odds that person 2 is not X?
A: 11/12 (Shannon had an 11 out of 12 chance of not being a Leo)
She could have been any number of non-Leo signs: Aries, Pisces, Sagittarius….. it doesn’t matter. But for the purpose of Step 2, we have to define a sign. Let’s go with Taurus.
2. In the second pair (children), what are the odds that person 1 is a Leo and person 2 is a Taurus?
A: We’ve already did this one…. it’s 1/12 * 1/12
The math is: 1 * 11/12 * 1/12 * 1/12
.917 * .083 * .083
The final answer is: .0063
Or, in English:
There was a 63/10,000 chance….
By the way, when I was trying to work this out, a Camp Happy citizen suggested I’d be good at poker. However, he was assuming I use Numbers and Probability to make decisions. Alas, as a Myers-Briggs INFP, I too often go by “instinct” and “feelings.” I’m trying to change that, though! What are the odds of me succeeding?