Algebra Puzzle Solution

Since,

3 pens = 12,

1 pen = 12÷3=4;

Since,

1 pen+2 guitars = 30,

2 guitars = 30-4=26
1 guitar = 26÷2=13;

Since,
1 pen+2 cars =18,
2 cars = 18-4=14
1 car = 14÷2=7