Question

Zoologists are studying two newly discovered species of insects in a previously unexplored section of rain forest. They estimate the current population of insect A to be 1.3 million and the current population of insect B to be 2.1 million. As development is encroaching on the section of rain forest where these insects live, the zoologists estimate the populations of insect A to be reducing at a rate of 3.8% and insect B to be reducing at a rate of 4.6%. If P represents the population of each species of insect in millions, and t represents the elapsed time in years, then which of the following systems of equations can be used to determine how long it will be before the populations of the two species are equal?

Answer 1

The population Pa of insect A after t years is given by the equation 
     Pa = 1.3(1-0.038)^t 
while the population Pb of insect B after t years is 
     Pb = 2.1(1-0.046)^t

We equate the above expressions to find the number of years t it will take the two populations to be equal: 
     Pa = Pb
     1.3(1-0.038)^t  = 2.1(1-0.046)^t
     1.3(0.962)^t  = 2.1(0.954)^t
These are the equations that can be used to determine how long it will be before the populations of the two species are equal.

We can now solve for t:
     (0.962)^t / (0.954)^t = 2.1/1.3
     (0.962/0.954)^t = 2.1/1.3
After taking the log of both sides of our equation, number of years t is
     t = log (2.1/1.3) / log (0.962/0.954)
     t = 57 years
Therefore, it will take 57 years for the population of insect A to equal the population of insect B.

Answer 2

For these types of problems, the equations of the form should be considered:
 y = A * (B) ^ t
 Where:
 A: initial population.
 B: annual exchange rate.
 T: Time in years.
 Then the equations of the system that must be used to find the time in which both are equal are:
 1, 3,00,000 * (1-0,038) ^ t for the type of insect A
 2, 1 00 000 * (1-0,046) ^ t for the type of insect B.