| 40 | m | + | ( | 80 | + | 60 | ) | ( | 12 | − | 2 | m | ) | + | 40 | = | 640 |
| 方程左边 = | 40 | m | + | 80 | ( | 12 | − | 2 | m | ) | + | 60 | ( | 12 | − | 2 | m | ) | + | 40 |
| = | 40 | m | + | 80 | × | 12 | − | 80 | × | 2 | m | + | 60 | ( | 12 | − | 2 | m | ) | + | 40 |
| = | 40 | m | + | 960 | − | 160 | m | + | 60 | ( | 12 | − | 2 | m | ) | + | 40 |
| = | - | 120 | m | + | 1000 | + | 60 | ( | 12 | − | 2 | m | ) |
| = | - | 120 | m | + | 1000 | + | 60 | × | 12 | − | 60 | × | 2 | m |
| = | - | 120 | m | + | 1000 | + | 720 | − | 120 | m |
| = | - | 240 | m | + | 1720 |
| - | 240 | m | + | 1720 | = | 640 |
| - | 240 | m | = | 640 | − | 1720 |
| - | 240 | m | = | - | 1080 |
| 1080 | = | 240 | m |
| 240 | m | = | 1080 |
| m | = | 1080 | ÷ | 240 |
| = | 1080 | × | 1 240 |
| = | 9 | × | 1 2 |
| m | = | 9 2 |
| m | = | 4.5 |