| 120 | ( | x | − | 500 | ) | + | 220 | ( | 900 | × | 21 20 | − | 600 | ) | = | 77700 |
| Left side of the equation = | 120 | x | − | 120 | × | 500 | + | 220 | ( | 900 | × | 21 20 | − | 600 | ) |
| = | 120 | x | − | 60000 | + | 220 | ( | 900 | × | 21 20 | − | 600 | ) |
| = | 120 | x | − | 60000 | + | 220 | × | 900 | × | 21 20 | − | 220 | × | 600 |
| = | 120 | x | − | 60000 | + | 207900 | − | 132000 |
| = | 120 | x | + | 15900 |
| 120 | x | + | 15900 | = | 77700 |
| 120 | x | = | 77700 | − | 15900 |
| 120 | x | = | 61800 |
| x | = | 61800 | ÷ | 120 |
| = | 61800 | × | 1 120 |
| = | 515 | × | 1 |
| x | = | 515 |