| ( | 20 | − | y | ) | ( | 300 | + | 20 | y | ) | = | 6500 |
| Left side of the equation = | 20 | ( | 300 | + | 20 | y | ) | − | y | ( | 300 | + | 20 | y | ) |
| = | 20 | × | 300 | + | 20 | × | 20 | y | − | y | ( | 300 | + | 20 | y | ) |
| = | 6000 | + | 400 | y | − | y | ( | 300 | + | 20 | y | ) |
| = | 6000 | + | 400 | y | − | y | × | 300 | − | y | × | 20 | y |
| = | 6000 | + | 100 | y | − | y | × | 20 | y |
| 6000 | + | 100 | y | − | y | × | 20 | y | = | 6500 |