| ( | 31 | − | 25 | ) | x | + | ( | 36 | − | 28 | ) | ( | 100 | − | x | ) | = | 720 |
| 方程左边 = | 31 | x | − | 25 | x | + | ( | 36 | − | 28 | ) | ( | 100 | − | x | ) |
| = | 6 | x | + | ( | 36 | − | 28 | ) | ( | 100 | − | x | ) |
| = | 6 | x | + | 36 | ( | 100 | − | x | ) | − | 28 | ( | 100 | − | x | ) |
| = | 6 | x | + | 36 | × | 100 | − | 36 | x | − | 28 | ( | 100 | − | x | ) |
| = | 6 | x | + | 3600 | − | 36 | x | − | 28 | ( | 100 | − | x | ) |
| = | - | 30 | x | + | 3600 | − | 28 | ( | 100 | − | x | ) |
| = | - | 30 | x | + | 3600 | − | 28 | × | 100 | + | 28 | x |
| = | - | 30 | x | + | 3600 | − | 2800 | + | 28 | x |
| = | - | 2 | x | + | 800 |
| - | 2 | x | + | 800 | = | 720 |
| - | 2 | x | = | 720 | − | 800 |
| - | 2 | x | = | - | 80 |
| 80 | = | 2 | x |
| 2 | x | = | 80 |
| x | = | 80 | ÷ | 2 |
| = | 80 | × | 1 2 |
| = | 40 | × | 1 |
| x | = | 40 |