| ( | y | − | 40 | ) | ( | 20 | + | 60 | − | y | ÷ | 1 2 | × | 1 | ) | = | 400 |
| Left side of the equation = | y | ( | 20 | + | 60 | − | y | ÷ | 1 2 | × | 1 | ) | − | 40 | ( | 20 | + | 60 | − | y | ÷ | 1 2 | × | 1 | ) |
| = | y | × | 20 | + | y | × | 60 | − | y | y | ÷ | 1 2 | × | 1 | − | 40 | ( | 20 | + | 60 | − | y | ÷ | 1 2 | × | 1 | ) |
| = | y | × | 20 | + | y | × | 60 | − | y | y | × | 2 | − | 40 | ( | 20 | + | 60 | − | y | ÷ | 1 2 | × | 1 | ) |
| = | 80 | y | − | y | y | × | 2 | − | 40 | ( | 20 | + | 60 | − | y | ÷ | 1 2 | × | 1 | ) |
| = | 80 | y | − | y | y | × | 2 | − | 40 | × | 20 | − | 40 | × | 60 | + | 40 | y | ÷ | 1 2 |
| = | 80 | y | − | y | y | × | 2 | − | 800 | − | 2400 | + | 80 | y |
| = | 160 | y | − | y | y | × | 2 | − | 3200 |
| 160 | y | − | y | y | × | 2 | − | 3200 | = | 400 |