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
