| ( | 20 | − | x | ) | ( | 400 | + | 50 | x | ) | − | 4800 | = | 0 |
| Left side of the equation = | 20 | ( | 400 | + | 50 | x | ) | − | x | ( | 400 | + | 50 | x | ) | − | 4800 |
| = | 20 | × | 400 | + | 20 | × | 50 | x | − | x | ( | 400 | + | 50 | x | ) | − | 4800 |
| = | 8000 | + | 1000 | x | − | x | ( | 400 | + | 50 | x | ) | − | 4800 |
| = | 3200 | + | 1000 | x | − | x | ( | 400 | + | 50 | x | ) |
| = | 3200 | + | 1000 | x | − | x | × | 400 | − | x | × | 50 | x |
| = | 3200 | + | 600 | x | − | x | × | 50 | x |
| 3200 | + | 600 | x | − | x | × | 50 | x | = | 0 |