| ( | 30 | + | x | ) | ( | 400 | − | 10 | x | ) | = | 2000 |
| Left side of the equation = | 30 | ( | 400 | − | 10 | x | ) | + | x | ( | 400 | − | 10 | x | ) |
| = | 30 | × | 400 | − | 30 | × | 10 | x | + | x | ( | 400 | − | 10 | x | ) |
| = | 12000 | − | 300 | x | + | x | ( | 400 | − | 10 | x | ) |
| = | 12000 | − | 300 | x | + | x | × | 400 | − | x | × | 10 | x |
| = | 12000 | + | 100 | x | − | x | × | 10 | x |
| 12000 | + | 100 | x | − | x | × | 10 | x | = | 2000 |