| ( | x | − | 4 | ) | × | 5 | + | 4 | × | 20 | = | ( | 20 | × | 4 | + | 5 | x | ) | × | 90 100 |
| 方程左边 = | ( | x | − | 4 | ) | × | 5 | + | 80 |
| ( | x | − | 4 | ) | × | 5 | + | 80 | = | ( | 20 | × | 4 | + | 5 | x | ) | × | 90 100 |
| 方程左边 = | x | × | 5 | − | 4 | × | 5 | + | 80 |
| = | x | × | 5 | − | 20 | + | 80 |
| = | 5 | x | + | 60 |
| 5 | x | + | 60 | = | ( | 20 | × | 4 | + | 5 | x | ) | × | 90 100 |
| 方程右边 = | 20 | × | 4 | × | 90 100 | + | 5 | x | × | 90 100 |
| = | 72 | + | 9 2 | x |
| 5 | x | + | 60 | = | 72 | + | 9 2 | x |
| 5 | x | − | 9 2 | x | = | 72 | − | 60 |
1 2 | x | = | 72 | − | 60 |
1 2 | x | = | 12 |
| x | = | 12 | ÷ | 1 2 |
| = | 12 | × | 2 |
| x | = | 24 |