| ( | 80 | − | 10 | x | ) | ( | 5 | x | + | 2 | ) | = | 1200 |
| Left side of the equation = | 80 | ( | 5 | x | + | 2 | ) | − | 10 | x | ( | 5 | x | + | 2 | ) |
| = | 80 | × | 5 | x | + | 80 | × | 2 | − | 10 | x | ( | 5 | x | + | 2 | ) |
| = | 400 | x | + | 160 | − | 10 | x | ( | 5 | x | + | 2 | ) |
| = | 400 | x | + | 160 | − | 10 | x | × | 5 | x | − | 10 | x | × | 2 |
| = | 400 | x | + | 160 | − | 50 | x | x | − | 20 | x |
| = | 380 | x | + | 160 | − | 50 | x | x |
| 380 | x | + | 160 | − | 50 | x | x | = | 1200 |