| ( | 320 | − | m | ) | ÷ | 1 | = | ( | 280 | − | m | ) | ÷ | 4 5 |
| Left side of the equation = | 320 | × | 1 | − | m | × | 1 |
| = | 320 | − | m | × | 1 |
| 320 | − | 1 | m | = | ( | 280 | − | m | ) | ÷ | 4 5 |
| Right side of the equation = | 280 | × | 5 4 | − | m | × | 5 4 |
| = | 350 | − | m | × | 5 4 |
| 320 | − | 1 | m | = | 350 | − | 5 4 | m |
| - | 1 | m | + | 5 4 | m | = | 350 | − | 320 |
1 4 | m | = | 350 | − | 320 |
1 4 | m | = | 30 |
| m | = | 30 | ÷ | 1 4 |
| = | 30 | × | 4 |
| m | = | 120 |