| ( | 40 | − | 5 | x | ) | ( | 20 | + | 10 | x | ) | = | 1200 |
| Left side of the equation = | 40 | ( | 20 | + | 10 | x | ) | − | 5 | x | ( | 20 | + | 10 | x | ) |
| = | 40 | × | 20 | + | 40 | × | 10 | x | − | 5 | x | ( | 20 | + | 10 | x | ) |
| = | 800 | + | 400 | x | − | 5 | x | ( | 20 | + | 10 | x | ) |
| = | 800 | + | 400 | x | − | 5 | x | × | 20 | − | 5 | x | × | 10 | x |
| = | 800 | + | 400 | x | − | 100 | x | − | 50 | x | x |
| = | 800 | + | 300 | x | − | 50 | x | x |
| 800 | + | 300 | x | − | 50 | x | x | = | 1200 |