| 420 | ÷ | ( | 420 | + | X | ) | = | 42 100 |
| Multiply both sides of the equation by: | ( | 420 | + | X | ) |
| 420 | = | 42 100 | ( | 420 | + | X | ) |
| 420 | = | 42 100 | × | 420 | + | 42 100 | X |
| 420 | = | 882 5 | + | 42 100 | X |
| - | 42 100 | X | = | 882 5 | − | 420 |
| - | 42 100 | X | = | - | 1218 5 |
1218 5 | = | 42 100 | X |
42 100 | X | = | 1218 5 |
| X | = | 1218 5 | ÷ | 42 100 |
| = | 1218 5 | × | 100 42 |
| = | 29 | × | 20 |
| X | = | 580 |