| 40 | ( | 1 | + | x | ) | + | 20 | x | + | 10 | ( | 1 | + | 2 | x | ) | = | 690 |
| 方程左边 = | 40 | × | 1 | + | 40 | x | + | 20 | x | + | 10 | ( | 1 | + | 2 | x | ) |
| = | 40 | + | 40 | x | + | 20 | x | + | 10 | ( | 1 | + | 2 | x | ) |
| = | 40 | + | 60 | x | + | 10 | ( | 1 | + | 2 | x | ) |
| = | 40 | + | 60 | x | + | 10 | × | 1 | + | 10 | × | 2 | x |
| = | 40 | + | 60 | x | + | 10 | + | 20 | x |
| = | 50 | + | 80 | x |
| 50 | + | 80 | x | = | 690 |
| 80 | x | = | 690 | − | 50 |
| 80 | x | = | 640 |
| x | = | 640 | ÷ | 80 |
| = | 640 | × | 1 80 |
| = | 8 | × | 1 |
| x | = | 8 |