A pond had an initial population of 120 fish. The number of fish is exponentially decreasing by one-fourth each year. If y represents the number of fish in the pond after x years, determine the graph of the solution set for this situation and the equation modeled by the graph.

1) Number of fish in the pond: y
Number of years: x

A pond had an initial population of 120 fish. Then when x=0, y=120. The graph must begin at point (x,y)=(0,120). Only the graph above at right and below at left begin in this point (0,120).

The number of fish is exponentially decreasing by one-fourth each year. Then the first year the number of fish must decrease 120*(1/4)=120/4=30, and the number of fish after the first year must be:
120-30=90=120(1-1/4)=120(4-1)/4=120(3/4). Then when x=1, y=90. The point is (x,y)=(1,90)
In the graph above at right when x=1, y is between 24 and 36. y=90 is not in this interval. then this graph is not the correct.
In the graph below at left when x=1, y is between 84 and 96. y=90 is in this inverval. Then this is the correct graph.

Answer: The graph of the solution set for this situation is the graph below at left.

2) The equation has the form:
y=y0(r)^x
Where y0 is the initial population and r is the rate of reduction. In this case:
y0=120 and
r=1-1/4=(4-1)/4→r=3/4
Then the equation modeled by the graph is:
y=120(3/4)^x

Answer: The equation modeled by the graph is that above at right:
y=120(3/4)^x