| ( | 10 | − | x | ) | × | 40 | = | ( | 15 | − | x | ) | × | 20 |
| Left side of the equation = | 10 | × | 40 | − | x | × | 40 |
| = | 400 | − | x | × | 40 |
| 400 | − | 40 | x | = | ( | 15 | − | x | ) | × | 20 |
| Right side of the equation = | 15 | × | 20 | − | x | × | 20 |
| = | 300 | − | x | × | 20 |
| 400 | − | 40 | x | = | 300 | − | 20 | x |
| - | 40 | x | + | 20 | x | = | 300 | − | 400 |
| - | 20 | x | = | 300 | − | 400 |
| - | 20 | x | = | - | 100 |
| 100 | = | 20 | x |
| 20 | x | = | 100 |
| x | = | 100 | ÷ | 20 |
| = | 100 | × | 1 20 |
| = | 5 | × | 1 |
| x | = | 5 |