| ( | 12 | − | y | ) | ( | 300 | + | 100 | y | ) | = | 4000 |
| Left side of the equation = | 12 | ( | 300 | + | 100 | y | ) | − | y | ( | 300 | + | 100 | y | ) |
| = | 12 | × | 300 | + | 12 | × | 100 | y | − | y | ( | 300 | + | 100 | y | ) |
| = | 3600 | + | 1200 | y | − | y | ( | 300 | + | 100 | y | ) |
| = | 3600 | + | 1200 | y | − | y | × | 300 | − | y | × | 100 | y |
| = | 3600 | + | 900 | y | − | y | × | 100 | y |
| 3600 | + | 900 | y | − | y | × | 100 | y | = | 4000 |