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This is a so-called Fermi question. It's difficult to give a precise answer, but it's possible to estimate it up to the order of magnitude (or several). A solution proceeds by making a series of reasonable assumptions, and it often happens that an overestimation in one of them is compensated by an underestimation in another.The number of grains of sand on (mainland) Spanish beaches (not counting sand which is

permanently covered with water)

According to this Wikipedia page, the length of the Spanish coast is about 5000 km = 5 * 10^6 m (I would have guessed 1000 km at first; oh well). Let's assume that the beach is on average 10 m wide and 20 cm deep. Then the total beach volume is 5 * 10^6 * 10 * 0.2 = 10^7 m^3. Also, let's assume a grain of send is 1 mm^3. Then the number of grains is 10^7 * 10^9 = 10^16. Of course, I could be way off.

Let's follow the solution of this classic Fermi question. There are about 5 * 10^5 people in Edinburgh (hmm, I would have guessed at least 10^6). Let's say an individual gets a haircut every 3 weeks on average (much more rarely for me). Then there are 5 * 10^5 haircuts in about 20 days, i.e., about 2 * 10^4 haircuts a day. Suppose a hairdresser can serve 4 clients an hour, which makes about 30 clients per day. Then it would take about 600 hairdressers.(a) The number of (working) hairdressers in Edinburgh

Finally, an anecdote about estimating quantities up to the order of magnitude. They say that the famous Russian physicist Lev Landau was getting his salary one day. Having received the cash, he moved aside and started carefully counting the money. Someone in the line to the cashier said, "Mr. Landau, you taught yourself that magnitudes in physics have sense only up to the order of magnitude." Landau replied: "Money is located in the exponent."