| ( | 400 | − | 50 | x | ) | ( | 8 | + | 4 | x | ) | = | 4800 |
| 方程左边 = | 400 | ( | 8 | + | 4 | x | ) | − | 50 | x | ( | 8 | + | 4 | x | ) |
| = | 400 | × | 8 | + | 400 | × | 4 | x | − | 50 | x | ( | 8 | + | 4 | x | ) |
| = | 3200 | + | 1600 | x | − | 50 | x | ( | 8 | + | 4 | x | ) |
| = | 3200 | + | 1600 | x | − | 50 | x | × | 8 | − | 50 | x | × | 4 | x |
| = | 3200 | + | 1600 | x | − | 400 | x | − | 200 | x | x |
| = | 3200 | + | 1200 | x | − | 200 | x | x |
| 3200 | + | 1200 | x | − | 200 | x | x | = | 4800 |