There are 2,100 bacteria in a circular petri dish. the dish has a radius of 40 millimeters. what is the approximate population density? (use 3.14 for π)

A petri dish is simply a circle; thus, we use the formula of the area of the circle which is πr^2. 
Given r = 40 mm
A = π(40)^2 = 5026.55 sq. mm

Population density = bacteria count / area
PD = 2,100 / 5026.55
PD = 0.417782 bacteria / mm sq.

Therefore, the answer is approximately 0.418 bacteria per square millimeter.

Determine the area of the circular petri dish through the equation, A = πr^2. Substituting the radius to the equation and solving for area gives, 5024 mm^2. The population density is obtained by dividing the population with the calculated area. Thus, the population density is approximately 0.418 bacteria per mm^2. 

The first one, i believe, would be correct. all the others have one thing in common, and that's that they have repeating numbers. in the second one, x and y have repeating 6's and 3's, and in the third, x and y have repeating 6's as well. in the last one, there are repeating 3's. the first one has no repeating numbers.