| ( | 80 | − | 10 | x | ) | ( | 80 | + | 4 | x | ) | = | 1000 |
| 方程左边 = | 80 | ( | 80 | + | 4 | x | ) | − | 10 | x | ( | 80 | + | 4 | x | ) |
| = | 80 | × | 80 | + | 80 | × | 4 | x | − | 10 | x | ( | 80 | + | 4 | x | ) |
| = | 6400 | + | 320 | x | − | 10 | x | ( | 80 | + | 4 | x | ) |
| = | 6400 | + | 320 | x | − | 10 | x | × | 80 | − | 10 | x | × | 4 | x |
| = | 6400 | + | 320 | x | − | 800 | x | − | 40 | x | x |
| = | 6400 | − | 480 | x | − | 40 | x | x |
| 6400 | − | 480 | x | − | 40 | x | x | = | 1000 |