| ( | 1 | + | m | ) | ( | 300 | + | 100 | m | ) | = | 420 |
| Left side of the equation = | 1 | ( | 300 | + | 100 | m | ) | + | m | ( | 300 | + | 100 | m | ) |
| = | 1 | × | 300 | + | 1 | × | 100 | m | + | m | ( | 300 | + | 100 | m | ) |
| = | 300 | + | 100 | m | + | m | ( | 300 | + | 100 | m | ) |
| = | 300 | + | 100 | m | + | m | × | 300 | + | m | × | 100 | m |
| = | 300 | + | 400 | m | + | m | × | 100 | m |
| 300 | + | 400 | m | + | m | × | 100 | m | = | 420 |