to write the equation, let's use P for total liters of Petrolyn oil, N for total liters of natural oil, and S for total liters of synthetic oil.  We can write two equations here:

S + N= P (the total liters of Petrolyn is equal to the sum of the number of liters of synthetic plus natural).

You didn't include a number for the ratio, so let's say Petrolyn contains X liters of natural oil for every Y liters of synthetic oil.  The ratio X/Y is always constant, so to simplify let's call this ratio C. So, we know that N/S is always equal to C. 

Now say we want to make B liters of Petrolyn oil, where B is a number we are given.  then, S + N = B. Now we have two equations and two variables.

1) S + N = B

2) N/S = C. 

Now we just need to solve for one variable and plug that solution into the other equation. Solve equation 2) for N to get N = S*C. Now plug this into equation 1.

We now have S + (S*C) = B.  We can factor this into S* ( 1+C) = B, then solve for S. S = B/(1+C).  Now just plug this solution back into N=S*C: N = B*C / (1+C)

We were given B and C at the beginning of the problem, so we're done!