Question 830695
Not well stated.  Maybe you mean something like this:


Two grades of milk are 2.5% butterfat and 5% butterfat.  How many gallons of each should be mixed to give 100 gallons of milk which is 4% butterfat?


The equations which would let you find the quantities of each grade of milk to use would be, if u is how much 2.5% fat and v is how much 5% fat milks,


{{{(2.5u+5.0v)/100=4}}}  and  {{{u+v=100}}}
Solve the system for u and v.


The lesson which will explain this more thoroughly is "Mixing two concentrations for known amount of mixture", which you find in the lessons link in my profile.