The perimeter is 30 units
Triangles ABC And DEF are not similar triangles.
Answer:

Step-by-step explanation:
The fish population after t years can be modeled by the following equation:

In which F(0) is the initial population and r is the constant rate of decay.
Year one the fish population was 18000
This means that 
In year three the fish population was 8000 fish.
Two years later, so 






So

This is the same as 3/18 x 21/7, which you can simplify to 1/6 x 3, which is 3/6 also a half. So the answer is 1/2