Question 185283
Let r = rockfish 
Let b = bluefish 
There are 24 fish in all. 
Then: 
r + b = 24 
2.5r + 8b = 137 
Here we have a system of two equations in two unknowns. 
Can you take it from here? 
If not, write back. 

Solving for r we get: 24 - b

Then, 2.5(24 - b) + 8b = 137

60 -2.5b + 8b = 137

-2.5b + 8b = 137 - 60

5.5b = 77

b = 77/5.5

b = 14

There are 14 bluefish.

So, if there are 24 fish in all, how many rockfish?

The obvious answer is 10 rockfish.

Understand?