| ( | 20 | + | 2 | x | ) | ( | 15 | + | 2 | x | ) | − | 20 | × | 15 | = | 1 | ÷ | 3 | × | 20 | × | 15 |
| 方程左边 = | ( | 20 | + | 2 | x | ) | ( | 15 | + | 2 | x | ) | − | 300 |
| ( | 20 | + | 2 | x | ) | ( | 15 | + | 2 | x | ) | − | 300 | = | 1 | ÷ | 3 | × | 20 | × | 15 |
| 方程左边 = | 20 | ( | 15 | + | 2 | x | ) | + | 2 | x | ( | 15 | + | 2 | x | ) | − | 300 |
| = | 20 | × | 15 | + | 20 | × | 2 | x | + | 2 | x | ( | 15 | + | 2 | x | ) | − | 300 |
| = | 300 | + | 40 | x | + | 2 | x | ( | 15 | + | 2 | x | ) | − | 300 |
| = | 0 | + | 40 | x | + | 2 | x | ( | 15 | + | 2 | x | ) |
| = | 40 | x | + | 2 | x | × | 15 | + | 2 | x | × | 2 | x |
| = | 40 | x | + | 30 | x | + | 4 | x | x |
| = | 70 | x | + | 4 | x | x |
| 70 | x | + | 4 | x | x | = | 1 | ÷ | 3 | × | 20 | × | 15 |
| 方程右边 = | 100 |
| 70 | x | + | 4 | x | x | = | 100 |