| ( | 800 | − | 4 | x | ) | ( | 40 | − | 2 | x | ) | = | 15200 |
| Left side of the equation = | 800 | ( | 40 | − | 2 | x | ) | − | 4 | x | ( | 40 | − | 2 | x | ) |
| = | 800 | × | 40 | − | 800 | × | 2 | x | − | 4 | x | ( | 40 | − | 2 | x | ) |
| = | 32000 | − | 1600 | x | − | 4 | x | ( | 40 | − | 2 | x | ) |
| = | 32000 | − | 1600 | x | − | 4 | x | × | 40 | + | 4 | x | × | 2 | x |
| = | 32000 | − | 1600 | x | − | 160 | x | + | 8 | x | x |
| = | 32000 | − | 1760 | x | + | 8 | x | x |
| 32000 | − | 1760 | x | + | 8 | x | x | = | 15200 |