| ( | 8 | + | x | − | 6 | ) | ( | 400 | − | 20 | x | ) | = | 2340 |
| 方程左边 = | 8 | ( | 400 | − | 20 | x | ) | + | x | ( | 400 | − | 20 | x | ) | − | 6 | ( | 400 | − | 20 | x | ) |
| = | 8 | × | 400 | − | 8 | × | 20 | x | + | x | ( | 400 | − | 20 | x | ) | − | 6 | ( | 400 | − | 20 | x | ) |
| = | 3200 | − | 160 | x | + | x | ( | 400 | − | 20 | x | ) | − | 6 | ( | 400 | − | 20 | x | ) |
| = | 3200 | − | 160 | x | + | x | × | 400 | − | x | × | 20 | x | − | 6 | ( | 400 | − | 20 | x | ) |
| = | 3200 | + | 240 | x | − | x | × | 20 | x | − | 6 | ( | 400 | − | 20 | x | ) |
| = | 3200 | + | 240 | x | − | x | × | 20 | x | − | 6 | × | 400 | + | 6 | × | 20 | x |
| = | 3200 | + | 240 | x | − | x | × | 20 | x | − | 2400 | + | 120 | x |
| = | 800 | + | 360 | x | − | x | × | 20 | x |
| 800 | + | 360 | x | − | x | × | 20 | x | = | 2340 |