Answer:
We need 9L from one chemical and 12L from the other.
Explanation:
What 3:12 means is that if you add 3L from one chemical(lets say A chemical) then you will have to add 12L from the other chemical (lets say B chemical) to make the compound.
Now Lets take that you added
L of chemical A to make 45L of the compound.
So if add 3L from chemical A you have to add 12L of chemical B, so if you add
L of A then how many liters of B do you need?
⇒

=

So you will need
L of chemical B.
Now we can write an equation for the total amount of compound B as,
⇒



Therefore,
We need 9L of chemical A, and
We need 36L of chemical B