| ( | 90 | − | x | ) | ( | 200 | + | 10 | x | ) | = | 6250 |
| Left side of the equation = | 90 | ( | 200 | + | 10 | x | ) | − | x | ( | 200 | + | 10 | x | ) |
| = | 90 | × | 200 | + | 90 | × | 10 | x | − | x | ( | 200 | + | 10 | x | ) |
| = | 18000 | + | 900 | x | − | x | ( | 200 | + | 10 | x | ) |
| = | 18000 | + | 900 | x | − | x | × | 200 | − | x | × | 10 | x |
| = | 18000 | + | 700 | x | − | x | × | 10 | x |
| 18000 | + | 700 | x | − | x | × | 10 | x | = | 6250 |