| 40 | + | 40 | ( | 1 | + | x | ) | + | 40 | ( | 1 | + | x | ) | ( | 1 | + | x | ) | = | 280 |
| Left side of the equation = | 40 | + | 40 | × | 1 | + | 40 | x | + | 40 | ( | 1 | + | x | ) | ( | 1 | + | x | ) |
| = | 40 | + | 40 | + | 40 | x | + | 40 | ( | 1 | + | x | ) | ( | 1 | + | x | ) |
| = | 80 | + | 40 | x | + | 40 | ( | 1 | + | x | ) | ( | 1 | + | x | ) |
| = | 80 | + | 40 | x | + | 40 | × | 1 | ( | 1 | + | x | ) | + | 40 | x | ( | 1 | + | x | ) |
| = | 80 | + | 40 | x | + | 40 | ( | 1 | + | x | ) | + | 40 | x | ( | 1 | + | x | ) |
| = | 80 | + | 40 | x | + | 40 | × | 1 | + | 40 | x | + | 40 | x | ( | 1 | + | x | ) |
| = | 80 | + | 40 | x | + | 40 | + | 40 | x | + | 40 | x | ( | 1 | + | x | ) |
| = | 120 | + | 80 | x | + | 40 | x | ( | 1 | + | x | ) |
| = | 120 | + | 80 | x | + | 40 | x | × | 1 | + | 40 | x | x |
| = | 120 | + | 80 | x | + | 40 | x | + | 40 | x | x |
| = | 120 | + | 120 | x | + | 40 | x | x |
| 120 | + | 120 | x | + | 40 | x | x | = | 280 |