A petri dish is simply a circle; thus, we use the formula of the area of the circle which is <span>πr^2.
Given r = 40 mm
A = </span>π(40)^2 = <span>5026.55 sq. mm
</span>
Population density = bacteria count / area
PD = 2,100 / <span>5026.55
PD = </span><span>0.417782 bacteria / mm sq.
Therefore, the answer is approximately 0.418 </span><span>bacteria per square millimeter.</span>
They are building blocks of geometry
We can set this problem up as an algebraic equation, which we can then solve to find our answer.
We start at -3. So we can start our equation with that:

However, this doesn't represent any change in temperature, just that we start at -3. To show change, we will use the variable
, where
stands for the number of hours. Each hour accounts for a change of -2.5 in degrees, meaning that we must multiply our
by -2.5. We can now add this term into our equation:

We are trying to find when we will reach -18 degrees, so we will set our entire equation equal to -18 and solve:



It will take us 6 hours, or choice A.