| ( | 7 | − | y | ) | ( | 400 | + | 40 | y | ) | = | 2080 |
| Left side of the equation = | 7 | ( | 400 | + | 40 | y | ) | − | y | ( | 400 | + | 40 | y | ) |
| = | 7 | × | 400 | + | 7 | × | 40 | y | − | y | ( | 400 | + | 40 | y | ) |
| = | 2800 | + | 280 | y | − | y | ( | 400 | + | 40 | y | ) |
| = | 2800 | + | 280 | y | − | y | × | 400 | − | y | × | 40 | y |
| = | 2800 | − | 120 | y | − | y | × | 40 | y |
| 2800 | − | 120 | y | − | y | × | 40 | y | = | 2080 |