Different schools do it differently, but most will look at the last test as the most determinative. A couple of schools will take your highest score in each section but that is a minority view. I am aware of no schools that average. Bear in mind that all schools see all the tests, so if eg you got a 25 and then a 30, your 30 is not going to be as strong as a person's who only took it once and scored a 30.