| 3 | x | = | 8000 | + | 7200 | + | ( | x | + | 400 | ) |
| Right side of the equation = | 15200 | + | ( | x | + | 400 | ) |
| 3 | x | = | 15200 | + | ( | x | + | 400 | ) |
| Right side of the equation = | 15200 | + | x | + | 400 |
| = | 15600 | + | x |
| 3 | x | = | 15600 | + | x |
| 3 | x | − | x | = | 15600 |
| 2 | x | = | 15600 |
| x | = | 15600 | ÷ | 2 |
| = | 15600 | × | 1 2 |
| = | 7800 | × | 1 |
| x | = | 7800 |