Question

To an order of magnitude, how many piano tuners are in New York City? The physicist Enrico Fermi was famous for asking questions like this on oral Ph.D. qualifying examinations. His own facility in making order of magnitude calculations is exemplified in Problem 45.48. (Explain what values you are using to make your estimate.)

Answer 1

Suppose that the population of NYC is 10,000,000, and there're average of 3 people per household
= 3.3 million HHs
and 1 in 10 HHs  has a piano
= 330,000 pianos

on average, a piano is tuned once a year
= 330,000 tuning per year
= 6,000 tunings per week

suppose a piano tuner works 5 days a week and handling 5 pianos  a day. That will be 25 pianos a week

hence, you need 6,000/25 = 240 tuners

hope this helps