| ( | 120 | + | x | ) | ÷ | ( | 120 | + | 40 | + | x | ) | = | 4 5 |
| Multiply both sides of the equation by: | ( | 120 | + | 40 | + | x | ) |
| ( | 120 | + | x | ) | = | 4 5 | ( | 120 | + | 40 | + | x | ) |
| 120 | + | x | = | 4 5 | ( | 120 | + | 40 | + | x | ) |
| 120 | + | x | = | 4 5 | × | 120 | + | 4 5 | × | 40 | + | 4 5 | x |
| 120 | + | x | = | 96 | + | 32 | + | 4 5 | x |
| 120 | + | x | = | 128 | + | 4 5 | x |
| x | − | 4 5 | x | = | 128 | − | 120 |
1 5 | x | = | 128 | − | 120 |
1 5 | x | = | 8 |
| x | = | 8 | ÷ | 1 5 |
| = | 8 | × | 5 |
| x | = | 40 |