Answer:
City A and city B will have equal population 25years after 1990
Step-by-step explanation:
Given
Let
years after 1990
population function of city A
population function of city B
<u>City A</u>
---- initial population (1990)
--- rate
<u>City B</u>
----- t = 10 in 2000
---- t = 20 in 2010
Required
When they will have the same population
Both functions follow exponential function.
So, we have:
![A_t = A_0 * (1 + r_A)^t](https://tex.z-dn.net/?f=A_t%20%3D%20A_0%20%2A%20%281%20%2B%20r_A%29%5Et)
![B_t = B_0 * (1 + r_B)^t](https://tex.z-dn.net/?f=B_t%20%3D%20B_0%20%2A%20%281%20%2B%20r_B%29%5Et)
Calculate the population of city A in 2000 (t = 10)
![A_t = A_0 * (1 + r_A)^t](https://tex.z-dn.net/?f=A_t%20%3D%20A_0%20%2A%20%281%20%2B%20r_A%29%5Et)
![A_{10} = 10000 * (1 + 3\%)^{10}](https://tex.z-dn.net/?f=A_%7B10%7D%20%3D%2010000%20%2A%20%281%20%2B%203%5C%25%29%5E%7B10%7D)
![A_{10} = 10000 * (1 + 0.03)^{10}](https://tex.z-dn.net/?f=A_%7B10%7D%20%3D%2010000%20%2A%20%281%20%2B%200.03%29%5E%7B10%7D)
![A_{10} = 10000 * (1.03)^{10}](https://tex.z-dn.net/?f=A_%7B10%7D%20%3D%2010000%20%2A%20%281.03%29%5E%7B10%7D)
![A_{10} = 13439.16](https://tex.z-dn.net/?f=A_%7B10%7D%20%3D%2013439.16)
Calculate the population of city A in 2010 (t = 20)
![A_t = A_0 * (1 + r_A)^t](https://tex.z-dn.net/?f=A_t%20%3D%20A_0%20%2A%20%281%20%2B%20r_A%29%5Et)
![A_{20} = 10000 * (1 + 3\%)^{20}](https://tex.z-dn.net/?f=A_%7B20%7D%20%3D%2010000%20%2A%20%281%20%2B%203%5C%25%29%5E%7B20%7D)
![A_{20} = 10000 * (1 + 0.03)^{20}](https://tex.z-dn.net/?f=A_%7B20%7D%20%3D%2010000%20%2A%20%281%20%2B%200.03%29%5E%7B20%7D)
![A_{20} = 10000 * (1.03)^{20}](https://tex.z-dn.net/?f=A_%7B20%7D%20%3D%2010000%20%2A%20%281.03%29%5E%7B20%7D)
![A_{20} = 18061.11](https://tex.z-dn.net/?f=A_%7B20%7D%20%3D%2018061.11)
From the question, we have:
and
![B_{10} = \frac{1}{2} * A_{10}](https://tex.z-dn.net/?f=B_%7B10%7D%20%3D%20%5Cfrac%7B1%7D%7B2%7D%20%2A%20A_%7B10%7D)
![B_{10} = \frac{1}{2} * 13439.16](https://tex.z-dn.net/?f=B_%7B10%7D%20%3D%20%5Cfrac%7B1%7D%7B2%7D%20%2A%2013439.16)
![B_{10} = 6719.58](https://tex.z-dn.net/?f=B_%7B10%7D%20%3D%206719.58)
![18061.11 = B_{20} * (1 + 20\%)](https://tex.z-dn.net/?f=18061.11%20%3D%20B_%7B20%7D%20%2A%20%281%20%2B%2020%5C%25%29)
![18061.11 = B_{20} * (1 + 0.20)](https://tex.z-dn.net/?f=18061.11%20%3D%20B_%7B20%7D%20%2A%20%281%20%2B%200.20%29)
![18061.11 = B_{20} * (1.20)](https://tex.z-dn.net/?f=18061.11%20%3D%20B_%7B20%7D%20%2A%20%281.20%29)
Solve for B20
![B_{20} = \frac{18061.11}{1.20}](https://tex.z-dn.net/?f=B_%7B20%7D%20%3D%20%5Cfrac%7B18061.11%7D%7B1.20%7D)
![B_{20} = 15050.93](https://tex.z-dn.net/?f=B_%7B20%7D%20%3D%2015050.93)
and
can be used to determine the function of city B
![B_t = B_0 * (1 + r_B)^t](https://tex.z-dn.net/?f=B_t%20%3D%20B_0%20%2A%20%281%20%2B%20r_B%29%5Et)
For: ![B_{10} = 6719.58](https://tex.z-dn.net/?f=B_%7B10%7D%20%3D%206719.58)
We have:
![B_{10} = B_0 * (1 + r_B)^{10}](https://tex.z-dn.net/?f=B_%7B10%7D%20%3D%20B_0%20%2A%20%281%20%2B%20r_B%29%5E%7B10%7D)
![B_0 * (1 + r_B)^{10} = 6719.58](https://tex.z-dn.net/?f=B_0%20%2A%20%281%20%2B%20r_B%29%5E%7B10%7D%20%3D%206719.58)
For: ![B_{20} = 15050.93](https://tex.z-dn.net/?f=B_%7B20%7D%20%3D%2015050.93)
We have:
![B_{20} = B_0 * (1 + r_B)^{20}](https://tex.z-dn.net/?f=B_%7B20%7D%20%3D%20B_0%20%2A%20%281%20%2B%20r_B%29%5E%7B20%7D)
![B_0 * (1 + r_B)^{20} = 15050.93](https://tex.z-dn.net/?f=B_0%20%2A%20%281%20%2B%20r_B%29%5E%7B20%7D%20%3D%2015050.93)
Divide
by ![B_0 * (1 + r_B)^{10} = 6719.58](https://tex.z-dn.net/?f=B_0%20%2A%20%281%20%2B%20r_B%29%5E%7B10%7D%20%3D%206719.58)
![\frac{B_0 * (1 + r_B)^{20}}{B_0 * (1 + r_B)^{10}} = \frac{15050.93}{6719.58}](https://tex.z-dn.net/?f=%5Cfrac%7BB_0%20%2A%20%281%20%2B%20r_B%29%5E%7B20%7D%7D%7BB_0%20%2A%20%281%20%2B%20r_B%29%5E%7B10%7D%7D%20%3D%20%5Cfrac%7B15050.93%7D%7B6719.58%7D)
![\frac{(1 + r_B)^{20}}{(1 + r_B)^{10}} = 2.2399](https://tex.z-dn.net/?f=%5Cfrac%7B%281%20%2B%20r_B%29%5E%7B20%7D%7D%7B%281%20%2B%20r_B%29%5E%7B10%7D%7D%20%3D%202.2399)
Apply law of indices
![(1 + r_B)^{20-10} = 2.2399](https://tex.z-dn.net/?f=%281%20%2B%20r_B%29%5E%7B20-10%7D%20%3D%202.2399)
--- (1)
Take 10th root of both sides
![1 + r_B = \sqrt[10]{2.2399}](https://tex.z-dn.net/?f=1%20%2B%20r_B%20%3D%20%5Csqrt%5B10%5D%7B2.2399%7D)
![1 + r_B = 1.08](https://tex.z-dn.net/?f=1%20%2B%20r_B%20%3D%201.08)
Subtract 1 from both sides
![r_B = 0.08](https://tex.z-dn.net/?f=r_B%20%3D%200.08)
To calculate
, we have:
![B_0 * (1 + r_B)^{10} = 6719.58](https://tex.z-dn.net/?f=B_0%20%2A%20%281%20%2B%20r_B%29%5E%7B10%7D%20%3D%206719.58)
Recall that: ![(1 + r_B)^{10} = 2.2399](https://tex.z-dn.net/?f=%281%20%2B%20r_B%29%5E%7B10%7D%20%3D%202.2399)
So:
![B_0 * 2.2399 = 6719.58](https://tex.z-dn.net/?f=B_0%20%2A%202.2399%20%3D%206719.58)
![B_0 = \frac{6719.58}{2.2399}](https://tex.z-dn.net/?f=B_0%20%20%3D%20%5Cfrac%7B6719.58%7D%7B2.2399%7D)
![B_0 = 3000](https://tex.z-dn.net/?f=B_0%20%20%3D%203000)
Hence:
![B_t = B_0 * (1 + r_B)^t](https://tex.z-dn.net/?f=B_t%20%3D%20B_0%20%2A%20%281%20%2B%20r_B%29%5Et)
![B_t = 3000 * (1 + 0.08)^t](https://tex.z-dn.net/?f=B_t%20%3D%203000%20%2A%20%281%20%2B%200.08%29%5Et)
![B_t = 3000 * (1.08)^t](https://tex.z-dn.net/?f=B_t%20%3D%203000%20%2A%20%281.08%29%5Et)
The question requires that we solve for t when:
![A_t = B_t](https://tex.z-dn.net/?f=A_t%20%3D%20B_t)
Where:
![A_t = A_0 * (1 + r_A)^t](https://tex.z-dn.net/?f=A_t%20%3D%20A_0%20%2A%20%281%20%2B%20r_A%29%5Et)
![A_t = 10000 * (1 + 3\%)^t](https://tex.z-dn.net/?f=A_t%20%3D%2010000%20%2A%20%281%20%2B%203%5C%25%29%5Et)
![A_t = 10000 * (1 + 0.03)^t](https://tex.z-dn.net/?f=A_t%20%3D%2010000%20%2A%20%281%20%2B%200.03%29%5Et)
![A_t = 10000 * (1.03)^t](https://tex.z-dn.net/?f=A_t%20%3D%2010000%20%2A%20%281.03%29%5Et)
and
![B_t = 3000 * (1.08)^t](https://tex.z-dn.net/?f=B_t%20%3D%203000%20%2A%20%281.08%29%5Et)
becomes
![10000 * (1.03)^t = 3000 * (1.08)^t](https://tex.z-dn.net/?f=10000%20%2A%20%281.03%29%5Et%20%3D%203000%20%2A%20%281.08%29%5Et)
Divide both sides by 10000
![(1.03)^t = 0.3 * (1.08)^t](https://tex.z-dn.net/?f=%281.03%29%5Et%20%3D%200.3%20%2A%20%281.08%29%5Et)
Divide both sides by ![(1.08)^t](https://tex.z-dn.net/?f=%281.08%29%5Et)
![(\frac{1.03}{1.08})^t = 0.3](https://tex.z-dn.net/?f=%28%5Cfrac%7B1.03%7D%7B1.08%7D%29%5Et%20%3D%200.3)
![(0.9537)^t = 0.3](https://tex.z-dn.net/?f=%280.9537%29%5Et%20%3D%200.3)
Take natural logarithm of both sides
![\ln(0.9537)^t = \ln(0.3)](https://tex.z-dn.net/?f=%5Cln%280.9537%29%5Et%20%3D%20%5Cln%280.3%29)
Rewrite as:
![t\cdot\ln(0.9537) = \ln(0.3)](https://tex.z-dn.net/?f=t%5Ccdot%5Cln%280.9537%29%20%3D%20%5Cln%280.3%29)
Solve for t
![t = \frac{\ln(0.3)}{ln(0.9537)}](https://tex.z-dn.net/?f=t%20%3D%20%5Cfrac%7B%5Cln%280.3%29%7D%7Bln%280.9537%29%7D)
![t = 25.397](https://tex.z-dn.net/?f=t%20%3D%2025.397)
Approximate
![t = 25](https://tex.z-dn.net/?f=t%20%3D%2025)