| ( | 4 | + | X | ) | ( | 500 | − | 100 | X | ) | = | 800 |
| Left side of the equation = | 4 | ( | 500 | − | 100 | X | ) | + | X | ( | 500 | − | 100 | X | ) |
| = | 4 | × | 500 | − | 4 | × | 100 | X | + | X | ( | 500 | − | 100 | X | ) |
| = | 2000 | − | 400 | X | + | X | ( | 500 | − | 100 | X | ) |
| = | 2000 | − | 400 | X | + | X | × | 500 | − | X | × | 100 | X |
| = | 2000 | + | 100 | X | − | X | × | 100 | X |
| 2000 | + | 100 | X | − | X | × | 100 | X | = | 800 |