Pre Algebra
posted by Ry .
In Wilbur's math class, his grade is based on the average score of six tests, each worth 100 points. Wilbur always worries about his grade. He knows that his average on the first four tests is 88.5. What is the lowest average he could get on his next two tests and still average 90 overall?

If you think about the 6 tests as one big test, there are 600 marks overall. Wilbur got 88.5 * 4 = 354 out of the first 400 marks.
Now, to get 90 overall, you need 0.9*600 = 540 out of 600 marks. Now, 540354 = 186, so that he must get 186 out of the next 200 marks. This translates to an mean of 186/2 = 93 for each of the last 2 tests.