| ( | x | − | 8 | ) | ( | 400 | − | 200 | ) | = | 640 |
| Left side of the equation = | x | ( | 400 | − | 200 | ) | − | 8 | ( | 400 | − | 200 | ) |
| = | x | × | 400 | − | x | × | 200 | − | 8 | ( | 400 | − | 200 | ) |
| = | 200 | x | − | 8 | ( | 400 | − | 200 | ) |
| = | 200 | x | − | 8 | × | 400 | + | 8 | × | 200 |
| = | 200 | x | − | 3200 | + | 1600 |
| = | 200 | x | − | 1600 |
| 200 | x | − | 1600 | = | 640 |
| 200 | x | = | 640 | + | 1600 |
| 200 | x | = | 2240 |
| x | = | 2240 | ÷ | 200 |
| = | 2240 | × | 1 200 |
| = | 56 | × | 1 5 |
| x | = | 56 5 |
| x | = | 11.2 |