Solution:
Given:
Using the population decrease formula,
[tex]\begin{gathered} y=ab^x \\ where:b=1-r.....................(decreasing\text{ population\rparen} \\ \\ Hence, \\ y=a(1-r)^x \end{gathered}[/tex]where:
[tex]\begin{gathered} a=25000 \\ r=\frac{20}{100}=0.2 \\ y=a(1-r)^x \\ y=25000(1-0.2)^x \\ y=25000(0.8)^x \end{gathered}[/tex]Therefore, the exponential function is;
[tex]y=25000(0.8)^x[/tex]