Question 980160
The description is not complete.  The price for the lower quality oil is needed.  You could choose a variable for the price of the lower quality oil and then solve the problem, only to obtain formulas for the two different qualities oil, but no VALUES are possible ---- only formulas with variables.


How much of the lower quality oil?
What is the price of the lower quality oil?
Your description is NOT complete, so the question cannot be answered.


Deeper thinking:
y, the price of lower quality oil

1 part of the 150 credit per liter;
How much of the y credit per liter?

x,  parts of the cheaper quality oil to add


{{{highlight_green((1*150+x*y)/(1+x)=120)}}}

Can a ratio of x to y be found?
{{{(150+xy)=120(x+1)}}}
{{{xy+150=120x+120}}}
{{{xy-120x=120-150}}}
{{{120x-xy=30}}}
No resolution


This is all that can be done.