A university student completed courses worth 3 credits. The student earned a total of 54 credits after completing 15 courses. How many courses worth 4 credits did the students complete?
Let's denote the number of courses worth 4 credits as x.
The total number of courses completed is 15, so we can write the equation:
x + 15 - x = 15
4x + 3*(15-x) = 54
4x + 45 - 3x = 54
x + 45 = 54
x = 54 - 45
x = 9
Therefore, the student completed 9 courses worth 4 credits.