| ( | 10 | + | x | ) | ( | 400 | − | 20 | x | ) | = | 0 |
| Left side of the equation = | 10 | ( | 400 | − | 20 | x | ) | + | x | ( | 400 | − | 20 | x | ) |
| = | 10 | × | 400 | − | 10 | × | 20 | x | + | x | ( | 400 | − | 20 | x | ) |
| = | 4000 | − | 200 | x | + | x | ( | 400 | − | 20 | x | ) |
| = | 4000 | − | 200 | x | + | x | × | 400 | − | x | × | 20 | x |
| = | 4000 | + | 200 | x | − | x | × | 20 | x |
| 4000 | + | 200 | x | − | x | × | 20 | x | = | 0 |