There are 2,100 bacteria in a circular petri dish. the dish has a radius of 40 millimeters. what is the approximate population density?

Population: 2,100 bacteria
petri dish radius : 40 millimeters

Area of a circle = π r²
A = 3.14 * (40mm)²
A = 3.14 * 1,600 mm²
A = 5,024 mm²

Population density is the measurement of the population per unit area or unit volume:

population / area  = 2,100 / 5,024mm² = 0.41799
 
The approximate population density is 0.42 bacteria per square millimeter

The correct answer would be "convenience"