| ( | 551 | + | 100 | x | ) | ÷ | ( | 7 | + | x | ) | = | 90 |
| Multiply both sides of the equation by: | ( | 7 | + | x | ) |
| ( | 551 | + | 100 | x | ) | = | 90 | ( | 7 | + | x | ) |
| 551 | + | 100 | x | = | 90 | ( | 7 | + | x | ) |
| 551 | + | 100 | x | = | 90 | × | 7 | + | 90 | x |
| 551 | + | 100 | x | = | 630 | + | 90 | x |
| 100 | x | − | 90 | x | = | 630 | − | 551 |
| 10 | x | = | 630 | − | 551 |
| 10 | x | = | 79 |
| x | = | 79 | ÷ | 10 |
| = | 79 | × | 1 10 |
| x | = | 79 10 |
| x | = | 7.9 |