Casper has some whipping cream that is 18\%18%18, percent butterfat and some milk that is 4\%4%4, percent butterfat. He wants to make a 500\,\text{ml}500ml500, start text, m, l, end text mixture of them that is 12\%12%12, percent butterfat. Here's a graph that shows a system of equations for this scenario where xxx is the volume of whipping cream he uses and yyy is the volume of milk he uses.

It depends on the subject for me. some exams are easier than others so i usually go through it and answer questions i know based on my general knowledge, then double check after. going through the lesson can a lot  because the exams can be  very lesson-specific, but sometimes they are ridiculously long so a lot of people don't take the time. i did find that, when i went through the lessons, i was making 100's on my exams vs strictly using + the internet and  i make 90's.