Brain teaser - babies born per year / month / day?

brainteaser Market sizing
New answer on Jan 02, 2020
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Anonymous A asked on Dec 30, 2019

Hi All,

Could someone help me with this brain teaser:

"How many babies were born yesterday in the US?"

How would the answer change if the question would ask for the same value on a monthly or a yearly basis?

Thanks a lot for your help!

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Content Creator
replied on Jan 02, 2020
BCG |NASA |20+ interviews with 100% success rate| 120+ students coached |GMAT expert 780/800 score

I would follow the following process:

  • US population: 330M
  • US population "per year": 330M / 80 years = 4.1 M / year Here you are making the hypothesis that the average life expectancy is 80 years
  • Babies born in a day: 4.1 M / 365 = 11 k

Then you can state some aspects that cour estimation and add more complexity. For example:

  1. You could consider that natality is going down, adding a factor to decrease your number
  2. You can consider the natality distribution within a year: historically most of the brths are between July and September

Hope it helps,

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Content Creator
replied on Dec 30, 2019
McKinsey | MBA professor for consulting interviews


  • US population: 320M
  • Life expectation: 80y
  • HP: Population equally spread for every age -> 0-1 years old: 320M/80 = 4M
  • Baby born in a day: 4M/365 = 11k


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replied on Dec 31, 2019
95% practise, 5% theory. We can make 2-3 real cases in a session or split the time 50:50 between case study and general background


My logic would be as follows:

- There are about 300 mil. people living in the US

- Average family has 2 children (for simplicity) - in other words 1 child per person per life

- People live in average 70 years - lets asume, the amount of people in various age groups is equally split (i.e. same amount of people in age 1, same in age 31, same in 51 etc.)

==> 300 / 70 / 1 = 4 mil. children per year

==> 4 mil. / 365 = 11 tis. children per day

if you check it with the real statistics - its actually a very good yearly guess

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Content Creator
replied on Dec 30, 2019
McKinsey | Awarded professor at Master in Management @ IE | MBA at MIT |+180 students coached | Integrated FIT Guide aut


I would go with:

- Total population of the US: ~330 M people

- Assume that people are equally distributed in the population pyramid from 0 to 90 years old. This makes 9 buckets of people (e.g., from 0 to 10, from 10 to 20, etc.) >> 330/9 = 36 M people in the 0-10 bucket.

- Since natality is going down and you know the pyramid is not evenly distributed, you can go from those 36 M people from 0-10 to 30 M people.

- Dividing 30 M people / 10 years, you obtain 3M people per year being born yearly in the last 10 years.

- 3M people / 365 days you get approximately 8 K babies being born daily.

Hope it helps!



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Luca gave the best answer


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