| ( | 30 | + | 40 | ) | × | 10 | + | ( | 30 | X | + | 40 | ) | × | 4 | = | 120 |
| 方程左边 = | 30 | × | 10 | + | 40 | × | 10 | + | ( | 30 | X | + | 40 | ) | × | 4 |
| = | 300 | + | 400 | + | ( | 30 | X | + | 40 | ) | × | 4 |
| = | 700 | + | ( | 30 | X | + | 40 | ) | × | 4 |
| = | 700 | + | 30 | X | × | 4 | + | 40 | × | 4 |
| = | 700 | + | 120 | X | + | 160 |
| = | 860 | + | 120 | X |
| 860 | + | 120 | X | = | 120 |
| 120 | X | = | 120 | − | 860 |
| 120 | X | = | - | 740 |
| X | = | - | 740 | ÷ | 120 |
| = | - | 740 | × | 1 120 |
| = | - | 37 | × | 1 6 |
| X | = | - | 37 6 |
| X | = | - 6.166667 |