Answer:
80,000
Step-by-step explanation:
First, the diameters of both must be in the same unit. Converting the pinhead diameter from millimeter to nanometer, we get:
nm
To get the number of polio viruses that fit at the head of the pin, divide the pin size by the amount of virus in nanometers:


This concludes that 80,000 polio viruses fit the pinhead.