| ( | 1200 | ( | x | ÷ | 10 | ) | − | 800 | ) | ÷ | 800 | × | 100 100 | = | 5 100 |
| Left side of the equation = | ( | 1200 | ( | x | ÷ | 10 | ) | − | 800 | ) | × | 1 800 |
| ( | 1200 | ( | x | ÷ | 10 | ) | − | 800 | ) | × | 1 800 | = | 5 100 |
| Left side of the equation = | 1200 | ( | x | ÷ | 10 | ) | × | 1 800 | − | 800 | × | 1 800 |
| = | 3 2 | ( | x | ÷ | 10 | ) | − | 1 |
| = | 3 2 | x | ÷ | 10 | − | 1 |
| = | 3 20 | x | − | 1 |
3 20 | x | − | 1 | = | 5 100 |
3 20 | x | = | 5 100 | + | 1 |
3 20 | x | = | 21 20 |
| x | = | 21 20 | ÷ | 3 20 |
| = | 21 20 | × | 20 3 |
| = | 7 | × | 1 |
| x | = | 7 |