| ( | 20 | + | y | ) | ( | 500 | − | 10 | y | ) | = | 12000 |
| Left side of the equation = | 20 | ( | 500 | − | 10 | y | ) | + | y | ( | 500 | − | 10 | y | ) |
| = | 20 | × | 500 | − | 20 | × | 10 | y | + | y | ( | 500 | − | 10 | y | ) |
| = | 10000 | − | 200 | y | + | y | ( | 500 | − | 10 | y | ) |
| = | 10000 | − | 200 | y | + | y | × | 500 | − | y | × | 10 | y |
| = | 10000 | + | 300 | y | − | y | × | 10 | y |
| 10000 | + | 300 | y | − | y | × | 10 | y | = | 12000 |