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.