How would you answer the brainteaser question:

How many cows are there in Canada?

I started by assuming:

  1. Canada population = 200 million
  2. Average Canadian consumes 500g of beef a week
  3. Average cow weight is 150 kg

So the answer is (200 million * 500 g) / 150 kg = 666666 cows.

I don't think this is a correct way to approach the question, because my answer is time-dependent (the second assumption is a weekly one).

How would you approach this question?

Thank you very much.


Your method gives an estimate for the number of cows consumed in one week. You would therefore need to estimate how long these cows live before being slaughtered, in order to estimate how many would be living at any one time - e.g. if cows are slaughtered at 1 year old, then there would be 52 weeks' worth of cows at any time.

I presume this is intended as a Fermi estimate, so I won't nitpick too closely, but 150 kg is way too low for the average weight of a cow, even allowing for not all of that weight being beef. You might also consider import/export and the existence of dairy cattle if you wanted a more precise estimate.

  • $\begingroup$ While estimates are indeed just estimates I'd also note that Canada's population could be estimated nearer its actual value of 36m (20m or 50m would be better) - 200m puts it at over half the population of the USA. (Another thing to note is that 500g per week would be want a beef hungry adult might eat, not an average person, it could come down quite a bit probably). $\endgroup$ – Jonathan Allan Jan 20 '18 at 22:08
  • $\begingroup$ @JonathanAllan whoops, I should definitely have caught that too! $\endgroup$ – Geoffrey Brent Jan 20 '18 at 22:42

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