| 40 | m | + | ( | 80 | + | 60 | ) | ( | 12 | − | m | ) | ÷ | 2 | + | 40 | = | 640 |
| Left side of the equation = | 40 | m | + | 80 | ( | 12 | − | m | ) | × | 1 2 | + | 60 | ( | 12 | − | m | ) | × | 1 2 | + | 40 |
| = | 40 | m | + | 40 | ( | 12 | − | m | ) | + | 30 | ( | 12 | − | m | ) | + | 40 |
| = | 40 | m | + | 40 | × | 12 | − | 40 | m | + | 30 | ( | 12 | − | m | ) | + | 40 |
| = | 40 | m | + | 480 | − | 40 | m | + | 30 | ( | 12 | − | m | ) | + | 40 |
| = | 0 | m | + | 520 | + | 30 | ( | 12 | − | m | ) |
| = | 0 | m | + | 520 | + | 30 | × | 12 | − | 30 | m |
| = | 0 | m | + | 520 | + | 360 | − | 30 | m |
| = | - | 30 | m | + | 880 |
| - | 30 | m | + | 880 | = | 640 |
| - | 30 | m | = | 640 | − | 880 |
| - | 30 | m | = | - | 240 |
| 240 | = | 30 | m |
| 30 | m | = | 240 |
| m | = | 240 | ÷ | 30 |
| = | 240 | × | 1 30 |
| = | 8 | × | 1 |
| m | = | 8 |