| ( | 60 | × | 80 | ) | ÷ | 2 | − | 20 | a | ÷ | 2 | = | 60 | × | 60 | − | ( | 60 | − | a | ) | × | 60 | ÷ | 2 |
| 方程左边 = | ( | 60 | × | 80 | ) | × | 1 2 | − | 10 | a |
| ( | 60 | × | 80 | ) | × | 1 2 | − | 10 | a | = | 60 | × | 60 | − | ( | 60 | − | a | ) | × | 60 | ÷ | 2 |
| 方程左边 = | 60 | × | 80 | × | 1 2 | − | 10 | a |
| = | 2400 | − | 10 | a |
| 2400 | − | 10 | a | = | 60 | × | 60 | − | ( | 60 | − | a | ) | × | 60 | ÷ | 2 |
| 方程右边 = | 3600 | − | ( | 60 | − | a | ) | × | 30 |
| 2400 | − | 10 | a | = | 3600 | − | ( | 60 | − | a | ) | × | 30 |
| 方程右边 = | 3600 | − | 60 | × | 30 | + | a | × | 30 |
| = | 3600 | − | 1800 | + | a | × | 30 |
| = | 1800 | + | 30 | a |
| 2400 | − | 10 | a | = | 1800 | + | 30 | a |
| - | 10 | a | − | 30 | a | = | 1800 | − | 2400 |
| - | 40 | a | = | 1800 | − | 2400 |
| - | 40 | a | = | - | 600 |
| 600 | = | 40 | a |
| 40 | a | = | 600 |
| a | = | 600 | ÷ | 40 |
| = | 600 | × | 1 40 |
| = | 15 | × | 1 |
| a | = | 15 |