| 40 | ( | X | − | 10 | − | X | × | 10 100 | ) | − | ( | 100 | + | 70 | ) | = | 150 |
| Left side of the equation = | 40 | X | − | 40 | × | 10 | − | 40 | X | × | 10 100 | − | ( | 100 | + | 70 | ) |
| = | 40 | X | − | 400 | − | 4 | X | − | ( | 100 | + | 70 | ) |
| = | 36 | X | − | 400 | − | ( | 100 | + | 70 | ) |
| = | 36 | X | − | 400 | − | 100 | − | 70 |
| = | 36 | X | − | 570 |
| 36 | X | − | 570 | = | 150 |
| 36 | X | = | 150 | + | 570 |
| 36 | X | = | 720 |
| X | = | 720 | ÷ | 36 |
| = | 720 | × | 1 36 |
| = | 20 | × | 1 |
| X | = | 20 |