| ( | 20 | + | x | ) | ( | 400 | − | 10 | x | ) | = | 0 |
| Left side of the equation = | 20 | ( | 400 | − | 10 | x | ) | + | x | ( | 400 | − | 10 | x | ) |
| = | 20 | × | 400 | − | 20 | × | 10 | x | + | x | ( | 400 | − | 10 | x | ) |
| = | 8000 | − | 200 | x | + | x | ( | 400 | − | 10 | x | ) |
| = | 8000 | − | 200 | x | + | x | × | 400 | − | x | × | 10 | x |
| = | 8000 | + | 200 | x | − | x | × | 10 | x |
| 8000 | + | 200 | x | − | x | × | 10 | x | = | 0 |