| 80 | ( | 1 | + | x | ) | ( | 1 | + | x | ) | = | 72 |
| Left side of the equation = | 80 | × | 1 | ( | 1 | + | x | ) | + | 80 | x | ( | 1 | + | x | ) |
| = | 80 | ( | 1 | + | x | ) | + | 80 | x | ( | 1 | + | x | ) |
| = | 80 | × | 1 | + | 80 | x | + | 80 | x | ( | 1 | + | x | ) |
| = | 80 | + | 80 | x | + | 80 | x | ( | 1 | + | x | ) |
| = | 80 | + | 80 | x | + | 80 | x | × | 1 | + | 80 | x | x |
| = | 80 | + | 80 | x | + | 80 | x | + | 80 | x | x |
| = | 80 | + | 160 | x | + | 80 | x | x |
| 80 | + | 160 | x | + | 80 | x | x | = | 72 |