| ( | 15 | − | m | ) | ( | 400 | + | 5 | m | ) | = | 4620 |
| Left side of the equation = | 15 | ( | 400 | + | 5 | m | ) | − | m | ( | 400 | + | 5 | m | ) |
| = | 15 | × | 400 | + | 15 | × | 5 | m | − | m | ( | 400 | + | 5 | m | ) |
| = | 6000 | + | 75 | m | − | m | ( | 400 | + | 5 | m | ) |
| = | 6000 | + | 75 | m | − | m | × | 400 | − | m | × | 5 | m |
| = | 6000 | − | 325 | m | − | m | × | 5 | m |
| 6000 | − | 325 | m | − | m | × | 5 | m | = | 4620 |