| ( | 40 | + | 10 | x | ) | ( | 20 | − | x | ) | = | 1400 |
| Left side of the equation = | 40 | ( | 20 | − | x | ) | + | 10 | x | ( | 20 | − | x | ) |
| = | 40 | × | 20 | − | 40 | x | + | 10 | x | ( | 20 | − | x | ) |
| = | 800 | − | 40 | x | + | 10 | x | ( | 20 | − | x | ) |
| = | 800 | − | 40 | x | + | 10 | x | × | 20 | − | 10 | x | x |
| = | 800 | − | 40 | x | + | 200 | x | − | 10 | x | x |
| = | 800 | + | 160 | x | − | 10 | x | x |
| 800 | + | 160 | x | − | 10 | x | x | = | 1400 |