| ( | 12 | − | x | ) | × | 40 | + | x | ( | 40 | ÷ | 2 | ) | = | 400 |
| 方程左边 = | 12 | × | 40 | − | x | × | 40 | + | x | ( | 40 | ÷ | 2 | ) |
| = | 480 | − | x | × | 40 | + | x | ( | 40 | ÷ | 2 | ) |
| = | 480 | − | 40 | x | + | x | × | 40 | ÷ | 2 |
| = | 480 | − | 40 | x | + | x | × | 20 |
| = | 480 | − | 20 | x |
| 480 | − | 20 | x | = | 400 |
| - | 20 | x | = | 400 | − | 480 |
| - | 20 | x | = | - | 80 |
| 80 | = | 20 | x |
| 20 | x | = | 80 |
| x | = | 80 | ÷ | 20 |
| = | 80 | × | 1 20 |
| = | 4 | × | 1 |
| x | = | 4 |