Question

A university student completed courses worth 3 credits and some courses worth 4

credits. The student earned a total of 54 credits after completing 15 courses. How
many courses worth 4 credits did the student complete?

Answer 1

9 courses for 4 credits.

Use a system of equations. Let’s say that the number of courses worth 3 credits is X and the number of courses worth 4 credits is Y.

The total number of courses the student took was 15, so X+Y=15.

Each X course was worth 3 credits each and each Y course was worth 4 credits each, for a total of 54, so 3X+4Y=54.

I’m going to take the first equation and solve it for X so I can plug it in to the second equation.

X=15-Y.

Now, plug it in to the second equation to solve for Y.

3(15-Y)+4Y=54

1. Distribute what’s inside the parenthesis.

45-3Y+4Y=54

2. Combine like terms.

45+Y=54

3. Subtract 45 from both sides.

Y=9