Bunuel wrote:

If in a certain population, people are born at an average rate of 9 people every 2 seconds and people die at an average rate of 3 people every 2 seconds, by approximately how many people does the population increase in 1 day?

A. 17,280

B. 25,920

C. 129,600

D. 259,200

E. 388,800

If 9 people are born and 3 people die every 2 seconds, then the rate of increase is (9-3)= 6 people every 2 seconds, \(\frac{6p}{2secs}=\frac{3p}{1sec}\)

Population increase in one day

\(\frac{3p}{1sec}*\frac{60secs}{1min}*\frac{60mins}{1hr}*\frac{24hrs}{1day}=\frac{259,200p}{1day}\)In one day: 259,200 people

For the arithmetic in the numerator, estimating and

using the doubling/halving trick is easier

\((3*60*60*24)\approx(75*3,600)\)

\(=(150*1,800)=(300*900)=270,000\)Answers are far apart. Only one answer is close to 270,000:

259,200

Answer D

*This explanation of how to do conversion problems by canceling units is excellent.
_________________

In the depths of winter, I finally learned

that within me there lay an invincible summer.

-- Albert Camus, "Return to Tipasa"