| ( | 429 | + | 100 | n | ) | ÷ | ( | 5 | + | n | ) | = | 90 |
| Multiply both sides of the equation by: | ( | 5 | + | n | ) |
| ( | 429 | + | 100 | n | ) | = | 90 | ( | 5 | + | n | ) |
| 429 | + | 100 | n | = | 90 | ( | 5 | + | n | ) |
| 429 | + | 100 | n | = | 90 | × | 5 | + | 90 | n |
| 429 | + | 100 | n | = | 450 | + | 90 | n |
| 100 | n | − | 90 | n | = | 450 | − | 429 |
| 10 | n | = | 450 | − | 429 |
| 10 | n | = | 21 |
| n | = | 21 | ÷ | 10 |
| = | 21 | × | 1 10 |
| n | = | 21 10 |
| n | = | 2.1 |