Csdrhrt

A tank had 20 clownfish and angelfish. 4 clownfish were sold and some angelfish were added to the tank such that the number of angelfish increased by 50%. After that, there were 18 fish in the tank altogether. How many clownfish were there in the tank at first?

How do I create an algebraic expression for this?

You have $c+a=20$.

The clownfish were sold, so c becomes $c-4$.

The angelfish population increases by $50%$, so $a$ become $1.50a$.

Now, we have $c-4+1.50a=18$.

In the beginning, there are a total of $20$ fish in the tank. Let's mark $C$= number of clownfish in the beginning and $A=$number of angelfish in the beginning. Now we know that
$$
C+A = 20
$$
Then 4 clownfish were sold, resulting in $C$ getting replaced by $C-4$. Then the number of angelfish is increased by $50~%$, meaning that $A$ is replaced by $frac{3}{2}A$. So at the end we have 18 fish
$$
(C-4) + frac{3}{2}A = 18
$$
Simplifying the equations, we get the following pair of equations:
$$
left{
begin{array}{cccc}
C &+ A &=& 20 \
2C &+ 3A &=& 44
end{array}
right.
$$
Do you know how to solve this now?