TITLE: Fun mathematical model to announce pregnancy to husband
QUESTION [9 upvotes]: My husband is a mathematical modeler and I would love to give him a model to announce to him that we're pregnant! He is a pharmacometrician and a neurologist and often uses R, PKPD, and Markov models, but also works avidly with other mathematics (though I wouldn't be able to tell you what). Is it possible to have some type of model/equation to equal pregnant/biological chemistry of being pregnant? My knowledge on these subjects are minute so any help would be greatly appreciated!

REPLY [2 votes]: A mother is $21$ years older than her son.
In six years' time the child will be $5$ times younger than her mother.
Question: Where is the father?
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Attention to the question: Where is the father?
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Attention to the question: Where is the father?
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Solution:
The boy is now ten years old.
The mother has today $Y$ years.
$\implies X + 21 = Y$
In $6$ years:
$5 (X + 6) = Y + 6$
So
$5X + 30 = X + 21 + 6$
$4X = -3$
$X = -\dfrac 34$
The boy is now $-\dfrac 34$ years, that is, $-9$ months.

So the father is ....