| ( | 280 | − | 4 | x | ) | ( | 80 | − | x | ) | ÷ | 2 | = | 280 | × | 400 | ÷ | 12 |
| Left side of the equation = | 280 | ( | 80 | − | x | ) | × | 1 2 | − | 4 | x | ( | 80 | − | x | ) | × | 1 2 |
| = | 140 | ( | 80 | − | x | ) | − | 2 | x | ( | 80 | − | x | ) |
| = | 140 | × | 80 | − | 140 | x | − | 2 | x | ( | 80 | − | x | ) |
| = | 11200 | − | 140 | x | − | 2 | x | ( | 80 | − | x | ) |
| = | 11200 | − | 140 | x | − | 2 | x | × | 80 | + | 2 | x | x |
| = | 11200 | − | 140 | x | − | 160 | x | + | 2 | x | x |
| = | 11200 | − | 300 | x | + | 2 | x | x |
| 11200 | − | 300 | x | + | 2 | x | x | = | 280 | × | 400 | ÷ | 12 |
| Right side of the equation = | 28000 3 |
| 11200 | − | 300 | x | + | 2 | x | x | = | 28000 3 |