| ( | X | + | 10 | ) | ( | 600 | − | 20 | X | ) | = | 8400 |
| Left side of the equation = | X | ( | 600 | − | 20 | X | ) | + | 10 | ( | 600 | − | 20 | X | ) |
| = | X | × | 600 | − | X | × | 20 | X | + | 10 | ( | 600 | − | 20 | X | ) |
| = | 600 | X | − | X | × | 20 | X | + | 10 | × | 600 | − | 10 | × | 20 | X |
| = | 600 | X | − | X | × | 20 | X | + | 6000 | − | 200 | X |
| = | 400 | X | − | X | × | 20 | X | + | 6000 |
| 400 | X | − | X | × | 20 | X | + | 6000 | = | 8400 |